She didn’t even notice their entire outdoor wardrobes spread out on their beds, or the fact that Becca looks like an inflatable burglar. All four of them stare at each other for a second and then collapse on their beds, screaming with laughter into their duvets. And realising they’re actually going.

At lights-out they’re in their beds like good little girls – if the prefect has to come back, she might be in a more observant mood. After the bell goes, the edgy giddiness starts to fade. Something else starts to show through.

They’ve never listened to the sounds of the school falling asleep before, not this way, ears stretched like animals’. At first the flickers are constant: a burst of giggles through the wall, a faraway squeal, a patter of slippers as someone runs to the toilet. Then they drift farther apart. Then there’s silence.

When the clock at the back of the main building strikes one, Selena sits up.

They don’t talk. They don’t flick on torches, or bedside lights: anyone going down the corridor would see the flicker through the glass above the transom. In the window the moon is enormous, more than enough. They strip off their pyjamas and stuff their pillows under their sheets, pull on final jumpers and coats, deft and synchronised as if they’d been practising. When they’re ready they stand by their beds, boots dangling from their hands. They look at each other like explorers in the doorway of a long journey, all of them caught motionless in the moment before one of them takes the first step.

‘If you weirdos are serious about this,’ Julia says, ‘let’s do it.’

No one leaps out at them from a doorway, no stair creaks. On the ground floor Matron is snoring. When Becca fits the key into the door to the main building, it turns like the lock’s been oiled. By the time they reach the maths classroom and Julia reaches up to the fastening of the sash window, they already know the watchman is asleep or on the phone and will never look their way. Boots on and out of the window, one two three four quick and slick and silent, and they’re standing on the grass and it’s not a game any more.

The grounds are still as a set for a ballet, waiting for the first shivering run of notes from a flute; for the light girls to run in and stop, poised perfect and impossible, barely touching the grass. The white light comes from everywhere. The frost sings high in their ears.