Walden swept briskly into the next segment of the lesson she’d planned, a formal introduction to the theoretical underpinnings of the triple array, with one eye on the clock. Five minutes. Seven. She’d meant to do some group Q-and-A to check their fundamentals before moving on to notation. Will kept glancing at the door. Ten minutes. Walden rapidly reorganised the lesson plan in her head. “All right, Will, Mathias, let’s have you two on the same table. I want you to collaborate and work through this problem sheet”—after a few years teaching you were always ready with spares of everything—“revising double arrays, before we carry on with the triples.”

“I think we did this one last year,” said Will, looking at the first question.

“You did. I still have your marks in my mark book. Let’s see if you’ve improved. I’m just going to check if everything’s quite all right.”

Provided you knew where they were, and you’d told them what they were supposed to be doing, it was acceptable in a pinch to leave sixth formers to get on with some work by themselves. If it had been a Year Seven who’d disappeared from Walden’s lesson, she’d have had to email the school office to go hunting—because if you left a classroom of Year Sevens unsupervised, you had only yourself to blame when they invented a fun game of Jumping On The Desk to Touch the Ceiling, No I Don’t Know Why It Broke or the ever-popular Throwing Our Shoes Out the Window, No I Don’t Know Where They Landed.

Corridor empty. The nearest girls’ toilets were round the corner in the Humanities block. Walden was giving herself five minutes to find Nikki and Aneeta, and if she couldn’t do it in that time, she was alerting the office to do a sweep.

It took two minutes. Nikki and Aneeta were not in the girls’ toilets, but Walden followed a hunch and knocked on the closed door of the disabled toilet next to it, and they were in there, curled up on the floor in the corner together like two sad kittens in smart skirts and blazers. Nikki had her head in her arms. “It’s okay,” Aneeta was saying, “it’s going to be okay, all right? Shh, shh, you’re okay.”

She was such a kind, thoughtful, good-hearted young person, and she was obviously doing her very best to support her friend. But to Walden’s experienced eye this looked like it was above a fellow teenager’s pay grade. Eye level, she thought, and crouched next to them. “Nikki?”

“She’s all right, Dr Walden, she just needs a moment,” said Aneeta protectively.

“Nikki, can you talk to me?”