We pad along the corridor. Screams behind us, echoing, bouncing off the walls like they're never going to die away. I'm so glad I'm not in the gym. It sounds insane, way worse than when we snuck out.

At the end of the corridor we turn right. There are bodies sprawled across the floor. Students like us, scratched, torn, bitten, bloody. Dead. As we edge past, eyeing them nervously in case they spring to life, I note that their heads have been cracked open, their brains scooped out. Except one, a small girl whose skull is intact. The same can't be said for her guts - they're all over the place.

"Wait," I whisper, stopping by the girl. I look for Cass. "Give me your knife."

"No one touches the knife but me," he says coldly.

"Fine," I snap. "Then get over here and be ready to stab her in the head if she stirs."

"What the hell are we waiting for?" Linzer snarls.

"I need to find out something."

"Who do you think you are?" she screeches. "Some sort of bloody - "

There's a creaking sound. Bones thrust through the tips of the dead girl's fingers, each at least half an inch long. Her lips shake and pull back over her teeth, which are growing and getting thicker. Her arms writhe, then she sits up and hisses at us, hunger in her eyes. I shriek and fall back. She dives after me. Hooks my shirt with her fingers. Tries to dig in.

As I scream again, Cass appears by my side and drives his knife into the side of the girl's head, all the way to the hilt. She shivers, eyes rolling. He works the blade around, digs it in and out several times. The girl falls away from me and goes still.

I force myself to my feet and tug up my shirt, wildly examining my flesh for scratches, heart beating hard. The others are staring at me suspiciously. Cass's eyes are narrow, his fingers tight on the handle of the knife.

"Nothing," I moan happily, exposing my stomach to them. "She didn't cut me. See?"

"You're lucky," Cass snorts.

"Now let's get the hell out of here, or do you want to study them some more?" Linzer sneers.

"I've confirmed what I wanted to." I point at the other corpses. "Brains. Like in the movies. If they eat your brain, or if it's destroyed, you're properly dead and there's no coming back. That's how we kill them."

"No shit, Sherlock," Cass says. "Now let's - "

La Lips screeches. A couple of bloodstained kids have stumbled into the corridor. Their eyes light up when they spot us and they stagger forward.

"Run!" I bark, and in a second we're racing past classrooms and corpses.

The zombies follow silently. I shouldn't look back but I can't help myself. I glance over my shoulder and spot them closing in. They're not smiling or leering. They don't pant either. They run expressionlessly, like robots. Only their eyes are alive.

One of the zombies grabs La Lips, who was struggling to keep up with the rest of us. She goes down with a yelp and it tears into her.

"Don't!" I shout at Copper as he stops to try to rescue her.

"I have to!" he yells and kicks at the zombie's head. The other one leaps on him. He bellows and lets fly with a flurry of punches. But the zombie pulls up Copper's shirt and bites into the soft flesh of his stomach. As Copper screams with pain and terror, the zombie rises, lips and teeth red, and comes after the rest of us again, leaving Copper to suffer, die and turn into one of the walking dead.

I want to help Copper and La Lips but I can't. They're finished. No time to feel sorry for them. If the zombie grabs hold, I'm done for too. So I leg it, trying not to think about the friends I'm leaving behind. The poor, doomed friends that I've lost.

As we come to where the corridor branches, we turn left, but I steal a glance right and then wish that I hadn't. There's another group of kids. Maybe they had the same idea as us and were making for the exit. But they've been set upon by a pack of zombies. They're trapped against a wall, dozens of transformed students holding them there, chewing through their skulls. The captured kids are screaming, sobbing, throwing up. All helpless. All damned.

A couple of zombies at the rear of the pack spot us and break away, joining the one who was already hot on our tail. They chase us down another corridor. The black kid whose name I don't know slips on a sliver of intestines. One of them is on him a moment later. He fights back manfully but the zombie bites, scratches and pushes him down. We press on and leave him.

More corpses. The floor is sticky with blood. We dash past a room. The door's open. I spot a teacher inside, pinned to the whiteboard by four zombies. They're eating her, two on her arms, two on her legs, working their way up to her torso. She's alive and sobbing softly, her fingers, lips and eyelids spasming.

We come to a set of stairs. A few bodies lie spread-eagle across the steps. We clamber over them and up. But we can't all fit at once. It's a tight squeeze. Elbows and curses fly as each of us struggles to be first to the top.

I'm next to Cass, the pair of us pushing forward, when he gives a cry of shock. I turn. The zombies have him. One's got his right leg, one the left. Both have bitten into his calves.

Cass screams and kicks at the zombies, but they hold firm. His eyes meet mine. He silently pleads with me to help him, do something, stop this. I shake my head numbly, then reach for his knife. He jerks it away from me.

"Please," I whisper. "It's no good to you now."

Cass snarls at me and starts stabbing at the zombies. I watch him for a couple of seconds, then back away slowly. Neither Cass nor the zombies pay any attention to me. They're locked in battle, but it's a fight that has only one of two possible outcomes. They tuck into his brain and he dies. Or they leave his skull alone and he becomes one of them. Either way, Cass is lost to us, so I wipe him from my thoughts as best I can and hobble up the stairs after the others.