Someone was coughing again.

The sound of it, wet and phlegmy, made Frankie cringe. She focused on the Geography textbook in front of her, studying the labelled diagram of a volcano like it was the most fascinating thing in the world. But each cough was a wet slap against her eardrums.

Frankie looked behind her at the cougher; Lesley Philips, hunched over her desk, her body trembling with each cough. A few other kids shot her irritated looks but Mrs Sanford, droning on at the front of the classroom, didn't seem to have noticed.

Lesley coughed again and Frankie's fingers tightened on her pen.

"She shouldn't have come in today," Beth whispered from Frankie's right side.

"It's gross," Frankie said, glaring back at Lesley. She felt mean; it wasn't Lesley's fault she was sick, but she should have stayed home. There was only a year and five months left before Frankie left school for good, and she couldn't wait. No more starched uniforms, teachers with heinous coffee breath, and bad cafeteria food - she couldn't fathom why anyone wouldn't jump at the chance to have a day off school. Especially on a Friday. That meant a three-day weekend.

"Why don't they send her home?" Beth whispered.

"I don't know -"

"Frances! Bethany!" Mrs Sanford's voice rang out and both girls jumped. Their teacher put her hands on her broad, plaid-skirted hips and faced them with a bulldog glare. "Something you two want to share with the rest of the class?"

Beth dropped her eyes to the desktop. Frankie shook her head.

"Good." Mrs Sanford fixed them with another stern look before turning her attention back to the lesson.

Behind them Lesley retched, wetness rasping in her throat. Frankie pulled a face, and edged her chair further forward, away from the other girl.

"Lesley? Are you alright?" Mrs Sanford finally noticed something was wrong.

Lesley gasped and coughed. Her chair scraped as she pushed herself away from her desk.

Frankie twisted in her seat, alarmed now. Lesley didn't sound like she had a cough; she sounded like she was about to vomit up her intestines.

Mrs Sanford crossed the room in three strides. Lesley leaped to her feet. Her face was pale and sweaty, eyes glazed. She coughed and retched again, bile spilling over her lips.

Everyone around her hastily moved away.

"Lesley?" Mrs Sanford reached out a hand to the sick girl.

Lesley lunged forward. Her hands scrabbled at the teacher, clawing at Mrs Sanford's face. "What are you . . .?" Mrs Sanford grabbed Lesley's wrists and restrained her. Lesley gasped and panted, her head lolling on her neck. Sweat soaked her hair, turning it into slick ropes. Drool and bile splashed down her chin.

"What's wrong with her?" Beth gasped.

Frankie didn't know. All she wanted was to put as much distance between her and Lesley as she could. She felt bad for the sick girl, but whatever Lesley had looked serious and Frankie didn't want to catch it. Not even for a day off school.

Lesley stopped fighting. She sagged in Mrs Sanford's grip, loudly gasping. The teacher steered her to the door.

"I need everyone to stay here." She fixed them with one of her glares but it wasn't as potent as normal. The wideness of her eyes suggested she was as shaken by what had happened as the rest of the class.

She guided Lesley out of the room, closing the door behind her.

Shocked silence reigned until someone giggled. It was a nervous sound, the reaction of someone who didn't know what else to do. The giggle was followed by another then voices broke out, a babble of discussion over what had just happened.

Beth gripped Frankie's hand. "That was horrible," she said softly.

"You're telling me." Frankie's eyes were fixed on Lesley's desk. Spit gleamed on its surface. She hoped someone would disinfect it before anyone else sat there. Then she felt guilty for worrying about spit on a desk when there was obviously something very wrong with Lesley.

"Do you think she's okay?" Beth said, looking at the door. She was still holding Frankie's hand, the tips of her fingers turning slightly red as she squeezed.

Frankie forced a smile. "I'm sure she's fine." It wasn't true - Lesley had looked really sick - but saying it made Beth's face brighten. Neither of them really knew Lesley but it was in Beth's nature to worry about anyone that might be in trouble.

One of the rowdier boys climbed on a table. "Class is out, guys," he crowed, pumping his fists in the air.

Frankie rolled her eyes. Mrs Sanford or someone else would be along any minute to restore order. But it was another ten minutes before anyone turned up; a stocky male teacher that Frankie knew vaguely by sight but not by name. He barked orders at them, restoring order, and picked up the lesson as if nothing had happened.

He didn't say a word about Lesley Philips.

"What do you think happened to her?" Beth asked as she stood in the cafeteria queue beside Frankie.

Frankie shrugged, reaching for a burger. Lukewarm grease soaked through the paper wrapping and coated her fingers, but it was better than the alternative - a vegetarian pasta dish that looked like roadkill. "They probably got her parents to come pick her up."

"Yeah, but do you think she's okay?"

Frankie glanced back at her friend. Beth was worried again, a little crease marring her forehead. She always looked young for her age but when she wore that expression she looked even younger.

If her hands weren't full, Frankie would have hugged her. "I'm sure she's fine, Beth. She's just got extreme flu or something. She'll be back on Monday."

Beth placed the questionable-looking pasta dish on her tray. "I hope so."

Frankie didn't point out that even if Lesley was back on Monday, and none the worse for wear, she wouldn't suddenly become friends with Beth. Neither Frankie or Beth had paid Lesley much attention in the years they'd been at school with her and it would take more than a bug to change that.

They found a table at the back of the cafeteria and sat down opposite each other. Beth tucked into her vegetarian thing with a zeal that looked genuine. Frankie and Beth had been best friends for years but sometimes Frankie really didn't understand the other girl.

Frankie bit into her burger. It was barely even lukewarm anymore. "So," she said, around a mouthful of something she wasn't convinced had come from a cow, "weekend plans?"

"Something at yours," Beth said.

"No parties," Frankie warned, holding up a finger. "My parents would kill me if they found out."

Her parents were spending a romantic weekend in Bath, and Frankie could probably have any traces of a party cleaned up before they got back. But parents had a way of knowing when their offspring had been up to something. They were trusting Frankie to live on her own while they were away and she didn't want to mess that up. If she did, she'd probably be eighteen before she was left home alone again.

Beth perked up. "Girlie night. We can do face masks and watch chick flicks and eat ice cream."

The ice cream bit sounded fun but face masks? Frankie had never been into that sort of thing but Beth loved it and Frankie usually went along with it to keep her friend happy.

"Sounds good," she said. "You -"

A spate of hacking coughs cut her off. A boy on the other side of the cafeteria stumbled back from his table. His face was white, harsh coughs wracking his body. Goose-bumps prickled Frankie's skin.

"Do you think he's got what Lesley's got?" said Beth. She sounded nervous.

"Looks like it."

"But what is it?"

"Probably a bad case of the flu." It was the second time she'd said it and it sounded less convincing this time.

"Do you think it's going around?"

Frankie's stomach churned and she put down her half-eaten burger. "I really hope not." She might not be planning any wild parties while her parents were away but she was looking forward to having the house to herself. She didn't want to waste all that lying ill in bed.

The boy lunged forward suddenly. He grabbed a girl from the nearest table and thrust his face close to hers as if he was going to kiss her. She screamed and shoved him away. He came at her again. His eyes gleamed with the same fevered light as Lesley's right before she'd thrown herself at Mrs Sanford.

Beth glanced at Frankie and Frankie got the impression they were both thinking the same thing - what kind of flu made a person do that?

Two teachers charged across the room and pulled the coughing boy away. He struggled against them, spit flowing down his chin. He looked crazed, like he wasn't in control of what he was doing.

Frankie instinctively moved her body so she was shielding Beth. The boy didn't look like he was going to break free from the two teachers but Frankie wasn't taking any chances. Beth would probably want to mop his fevered brow or something.

The boy was pulled from the cafeteria, leaving behind a nervous buzz of voices. The people who'd seen what happened to Lesley looked the most nervous, and small wonder. Frankie had passed Lesley's condition off as a freak reaction to a common illness. Seeing it twice was unnerving.

Frankie pushed her burger away. The pink meat stared back at her like a grin, grease sliding over the edges of the bun. It made her think of the bile trickling from Lesley's mouth.

Beth swivelled back to her lunch. Her face was pale, her eyebrows pulled up to her blonde hairline. She didn't say anything, just prodded her lunch with a fork. Apparently Frankie wasn't the only one who'd lost her appetite.

"So . . . girlie night," Frankie said, trying to pick up where the conversation had left off. "We should see if Melly and Allison want to join us."

Beth nodded half-heartedly but her eyes kept straying to the door where the boy had been taken away. Frankie wanted to reassure her but the words stuck in her throat. She didn't know what she'd just witnessed but it wasn't normal. Flu didn't make people act like that.

Beth mumbled something about facemasks and Frankie pulled her mind to the weekend. But she couldn't help tensing every time she heard someone cough or clear their throat, and she couldn't help glancing over her shoulder to see who the offender was and whether or not they were about to go crazy and attack someone.

Lunchtime passed without further incident but there was a knot of unease in Frankie's stomach, sitting alongside half a burger. If two people had whatever that sickness was then there could easily be more. If it was contagious then any one of them could have it.

Frankie didn't feel sick but she was more anxious than ever for the school day to end so she could get away from whatever was going around.

At last three o'clock rolled around, signalling the end of the day and the end of the school week. There was always a certain kind of joy running through the air when pupils left school each day but it was strongest on a Friday. It was a whole weekend before anyone had to sit in a classroom again, and the freedom was something everyone revelled in. Even the teachers had more of a spring in their step.

Melly and Allison fell into step with Frankie and Beth as they exited the building. Melly had been friends with Frankie almost as long as Beth, and Allison had joined their group when she transferred to the school a year earlier. They weren't part of the popular clique nor were they outsiders, easy targets for bullies. They were just normal kids, not the first to be invited to parties but not the last either.

"So, you've got a free house this weekend, right, Frankie?" Allison said, pushing frizzy red hair off her face.

"No parties," Frankie warned.

Allison's face fell. Her frizz of hair flopped forward again. It had only taken one visit to Allison's house for Frankie to realise how protective Allison's parents were of her and her little brother. It was like they forgot Allison was fifteen, not five. Parties were something they strongly disapproved of and as result Allison rarely got to go to them. It wasn't surprising that Allison hoped a free house meant Frankie would throw a party Allison could actually go to.

Frankie nudged the other girl. "Sorry, but my parents would kill me."

Allison looked mournfully back at her through a tangle of hair. "You're lucky your parents even let you stay home alone. Mine never would."

Beth filled them in on the plan.

Melly's round face brightened at the mention of ice cream. "There's a two for one Ben and Jerry's offer in Tesco's," she said, patting her stomach. Her blue school-shirt had fitted her at the beginning of the year but now the buttons were starting to strain across her waistline. "Not all the flavours are on offer but they've definitely got the chocolate brownie and the cookie dough."

"How do you know?" Allison asked.

"Hogzilla's been scoping out the freezers since the offer came on," said a voice behind them.

Frankie stiffened. She and her friends weren't obvious targets for bullying but when it came to Vanessa Gilmore and her posse, everyone was a target. Vanessa strode past them, flinging glossy black hair over her shoulder with a manicured hand. The look she gave Melly was pure disgust.

"Ethically you shouldn't even be sold ice cream. Obesity's on the increase and you're one big fat statistic," she sneered.

Melly's face went pink and her lower lip wobbled. She wasn't sensitive about her weight until someone mocked her for it.

"Just ignore her, Melly," Frankie said loudly. She slowed her pace so Vanessa and her little gang of Vanessa-clones could sashay past, their high heels clicking on the pavement. There was a maximum heel height allowed at the school and Vanessa always seemed able to flout it. Frankie hoped that sooner or later Vanessa would trip in her stupid shoes and twist her stupid ankle. Maybe even hit her face on the pavement and end up with her stupid jaw wired shut.

One of the Vanessa-clones looked back at them. Frankie wasn't sure which it was - Jess or maybe Becky, they all looked the same to her. The girl whispered something to Vanessa. They both looked back at Frankie and giggled. Frankie just rolled her eyes. That was probably the highlight of the Vanessa-clone's day, being able to elicit a smile of approval from her glossy-haired fuehrer. It was hard to feel intimidated by someone so pathetic. At least it was for Frankie. Melly was another story.

They made their way to the school bus. The seats at the back - considered the best ones - were already taken, occupied by the Vanessa-gang. The queen bee herself sat in the middle of her cronies, her school skirt - two inches shorter than uniform regulation - arranged neatly across her knees.

Frankie gestured to some seats at the front of the bus. The more distance between Vanessa and Melly the better. She made sure Melly and Allison sat in front of her and Beth, as if that would somehow deflect any insults Vanessa might throw their way.

"What films are we thinking?" Allison asked, twisting in her seat so she could look at Frankie. "Mean Girls is a classic, or we could have a Sex and the City marathon."

"Bring whatever you want and we can decide on the night," Frankie said.

She looked forward to a night with her girls but, if she was honest, she also looked forward to a night alone tonight. It was the first time her parents had left her alone for a whole night and the novelty was exciting. She could pretend the whole house was hers, eat junk food, have a bath with the door open, play her music really loud and dance like a lunatic, and know that no one was going to walk in on her or tell her to stop doing something.

Her stomach churned with excitement.

The bus pulled away from the curb and the school faded into the distance.

"Let the weekend begin," someone yelled and the bus erupted into cheers. The bus driver, glancing in his rear view mirror, smiled indulgently at them.

Frankie didn't notice that the roads were unusually clear that day. Usually the rush of parents and buses picking kids up from school meant the roads were clogged with traffic.

She didn't notice a gaggle of kids walking home with confused expressions, mobile phones pressed to their ears as they tried to find out why their parents hadn't come to collect them.

She didn't notice the man in the business suit bent double at the side of the road, his body heaving with violent coughs.

But she did notice when something slammed into the side of the bus.