We head for the exit, numb, and dumb with shock. I don't think anyone now expects it to be open. But we act as if we hadn't seen the slaughter in the cafeteria, as if we don't know what it means.

We're at the door a minute later. We stare at the bar. If it works, and the door opens, we're just seconds away from freedom.

Nobody reaches for the bar. Everyone's afraid of being the one to fail, to dash the hope that we all long for but don't dare believe in. Finally I sigh and step up to the challenge. I push the bar down. It clicks. I pause a second, then push.

Nothing happens.

I close my eyes and lean my head against the door. Then I curse and push again, straining, putting everything into it. But I'm wasting my time. It doesn't budge.

"B," Trev says.

I fire one of my vilest curses his way.

"B," he says calmly. "Look."

I turn and spot something on the floor to our left. I saw it before but thought it was just a corpse. Now I see that there are actually two bodies. And one of them's moving, chewing on the head of the other, slurping down brains.

As we stare with disgust, the zombie pushes its victim away and stands. We all gasp at the same time.

"Mrs. Reed!" I shout.

The zombie that was once our principal sways from side to side, staring at us blankly, chin drenched with blood and flecked with bits of brain. I get a fix on the body beneath her and cringe. It's Jonesenzio. He won't be boring anyone with dry history lectures again. Poor old sod.

Mrs. Reed shuffles towards us. Nobody moves. She doesn't seem to pose an immediate threat. She's smiling stupidly, eyes unfocused, rubbing her stomach. She burps and giggles softly.

"This is unreal," the Indian kid sighs.

Mrs. Reed's eyes settle on him and she frowns. She raises a finger and shakes it slowly. Then she spots me. Her smile spreads again.

"Beeeeeeee," she wheezes.

"Bloody hell!" Elephant yelps. "How's she talking? Zombies can't talk! Can they?"

Mrs. Reed comes closer. She's within touching distance of me now but I can't move. I'm rooted. The others back up but nobody runs or screams or tries to pull me away from her. They're mesmerized, held captive by the spectacle like I am.

Mrs. Reed strokes my cheek with a finger - there's no bone sticking out of it - and leaves a trail of blood across my flesh. But she doesn't scratch me and claim me for one of her own. Her eyes are locked on mine. She looks demented but strangely peaceful at the same time.

"Fullll," she whispers, still rubbing her stomach with her other hand.

"What's happening?" Seez asks, distracting her. "Where did the zombies come from? Who are the freaks in the hoodies? Who locked the doors?"

Mrs. Reed snarls at him. Then she smiles at me again and taps the side of my head. "Stayyy. Hungry again... sooooon."

"Sorry," I croak, stepping away from her. "I don't fancy being eaten."

Mrs. Reed looks disappointed, but she shrugs and sits down. Dabs at the bits of brain stuck to her chin and sucks them from her finger.

I'm backing away from the zombie principal when I stop. This isn't right. She's not like any of the others we've encountered. And it's not just the fact that she can talk and doesn't have bones sticking out of her fingers. There are no bite marks or scratches. I can't see where she was wounded.

I want to study her properly - this seems important - but Trev interrupts.

"We have to get out of here."

"But this is weird," I argue. "She's different. I want to know why."

Trev shrugs. "Then stay and have a chat with her. Me, I'm heading for the front of the building, to get the hell out. They might have barred the doors but they can't have blocked all the windows. There wasn't enough time."

"He's right," Seez says. "The windows are our best hope."

"You don't have any hope," someone snickers behind us.

I whirl and spot three people in hoodies. They're spread across the corridor, grinning viciously. I'm almost certain that the one in the middle is the louse who tried to steal the baby in the museum. Then he points at me and says, "You should have let me take the boy," and my suspicions are confirmed.

"Who are you?" I yell. "Why are you doing this?"

"Don't worry," the mutant chuckles, his voice gravelly and gurgly, nothing like a normal person's. "You're in good company. This is happening all over London. This will be a city of zombies by the time the sun sets. And it won't be the only one. From tomorrow, this world is ours."

As we stare at the mutant with the crazy skin and yellow eyes, horrified by his prediction, he puts his whistle to his lips and blows. The others blow their whistles too. Three long, sharp toots. They're so piercing, I have to cover my ears with my hands. Then the mutants drop the whistles and smirk. Lowering my hands, I fix on the sound of a flurry of feet stomping down the corridor, dozens of zombies responding to the call of the mutants, closing in on us.