And there it was: the hand hesitating above the green folder. The extra second when she kept her face turned down to the bag, away from me, ponytail tumbling to hide her. The nerves I’d been watching for. Not ice-cream-cool and smooth right through, after all.

Then she straightened and met my eyes again, blank-faced. Her hand came up, held out the green folder. Let go as soon as I touched it, so quick I almost let it fall.

‘This was on the board.’

The folder said ‘Holly Mackey, 4L, Social Awareness Studies’, scribbled over. Inside: clear plastic envelope. Inside that: a thumbtack, fallen down into one corner, and a piece of card.

I recognised the face faster than I’d recognised Holly’s. He had spent weeks on every front page and every TV screen, on every department bulletin.

This was a different shot. Caught turning over his shoulder against a blur of autumn-yellow leaves, mouth opening in a laugh. Good-looking. Glossy brown hair, brushed forward boyband-style to thick dark eyebrows that sloped down at the outsides, gave him a puppydog look. Clear skin, rosy cheeks; a few freckles along the cheekbones, not a lot. A jaw that would’ve turned out strong, if there’d been time. WideMain Body Indented grin that crinkled his eyes and nose. A little bit cocky, a little bit sweet. Young, everything that rises green in your mind when you hear the word young. Summer romance, baby brother’s hero, cannon-fodder.

Glued below his face, across his blue T-shirt: words cut out of a book, spaced wide like a ransom note. Neat edges, snipped close.

I know who killed him

Holly watching me, silent.

I turned the envelope over. Plain white card, the kind you can buy anywhere to print off your photos. No writing, nothing.

I said, ‘Did you touch it?’

Eyes to the ceiling. ‘Course not. I went into the art room and got that’ – the envelope – ‘and a balsa knife. I pulled out the tack with the knife, and I caught the card and the tack in the envelope.’

‘Well done. And then?’

‘I put it up my shirt till I got back to my room, and then I put it in the folder. Then I said I felt sick and went back to bed. After the nurse came round, I sneaked out and came here.’

I asked, ‘Why?’

Holly gave me an eyebrows-up stare. ‘Because I thought you guys might want to know. If you don’t care, then you can just throw it away, and I can get back to school before they find out I’m gone.’

‘I care. I’m only delighted you found this. I’m just wondering why you didn’t take it to one of your teachers, or your dad.’