Meaning our girl wouldn’t have known we could narrow it down. She would have thought she could vanish, into the stream of bright faces tumbling down that corridor.

Conway said, ‘Were any of the girls in the main school after classes ended?’

Silence again. Then: ‘As you may know, Transition Year – fourth year – involves large amounts of practical work. Group projects. Experiments. So forth. Often, fourth-years’ homework requires access to school resources. The art room, the computers.’

Conway said, ‘Meaning there were fourth-years here yesterday evening. Who and when?’

The full-on headmistress stare. Full-on cop stare coming back. Miss McKenna said, ‘Meaning no such thing. I have no knowledge of who was in the main building yesterday. The matron, Miss Arnold, holds a key to the door connecting the school to the boarders’ wing, and makes a note of any girl who is given permission to enter the main building after hours; you would need to ask her. I am simply telling you that, on any given evening, I would expect at least a few fourth-years to be here. I understand that you feel the need to find sinister meaning everywhere, but believe me, Detective Conway, there will be nothing sinister about some poor child’s Media Studies project.’

‘That’s what we’re here to find out,’ Conway said. She stretched, big, back arching, arms going over her head and out. ‘That’ll do for now. We’ll need a list of girls who had access yesterday after school. Fast. Meanwhile, we’re taking a look at this invaluable board.’

She flipped the pen back onto the desk, neat snap of her wrist like skimming a stone. It rolled across the green leather, stopped an inch from Miss McKenna’s clasped hands. Miss McKenna didn’t move.

The school had gone quiet, the kind of quiet made out of a hundred different low buzzes. Somewhere girls were singing, a madrigal: just snippets, layered up with sweet high harmonies, cut off and started over every couple of lines when the teacher corrected something. Now is the month of maying, when merry lads are playing, fa la la la la . . .

Conway knew where we were headed. Top floor, down the corridor, past closed classroom doors (If tall dominates short, then . . . Et si nous n’étions pas allés . . . ). Open window at the end of the corridor, warm breeze and green smell pouring in.

‘Here we go,’ Conway said, and turned in to an alcove.

The board was maybe six foot across by three high, and it came leaping out of that alcove screaming straight in your face. Like a mind gone wrong, someone’s huge mad mind racketing out every-coloured pinballs full speed, with no stop button. Every inch of it was packed: photos, drawings, paintings, jammed in on top of each other, punching for space. Faces blacked out with marker. Words everywhere, scribbled, printed, sliced.

A sound from Conway, quick breath through her nose that could’ve been a laugh or the same shock.