‘Who was it, then?’ Forest asked. ‘We can’t trace it back to a person. Whoever it was must have set up a new staff account under a fake name, because it doesn’t tell us who you were sending messages to.’

But … she signed, then stopped.

‘But what, Cassie?’ Singh asked eventually.

Cassie looked at Sasha again. I don’t want to get into trouble. And I don’t want anyone else to get into trouble.

Tell them the truth, Cassie. Everything, Sasha told her.

Fine, the girl said with a sigh. My friend told me I had to pretend I’d been texting someone called Joe or Jo. They said it was all part of the game.

‘Who is your friend?’ Forest asked, her patience clearly wearing thin. I remembered the times before when Cassie had alluded to her friend, and comments from other students about this friend being imaginary. I also remembered Sasha’s concerns, when she thought someone was manipulating the students – we had been sure it was Steve, but whoever Cassie had been getting messages from was at the school before Jane was fired, so Steve couldn’t have been Joe after all. Who had been controlling her?

I can’t tell you, Cassie signed, her eyes wide. They told me never to tell anyone about them. I can’t.

The detectives looked at each other, then made an unspoken decision to try another tactic.

‘You say this friend of yours was pretending to be Joe, sending messages to Leon. Is that right?’ Singh asked.

Cassie nodded. And the others. Not just Leon.

‘Well, some of the first messages that “Joe” sent to Leon were texts to his new phone. The messages were backed up to an email address at the school.’

Oh, I know. It was Mr Wilkinson’s. My friend told me they could back up those messages, because then if they got caught we could pretend it was Mr Wilkinson. They said it wouldn’t be a problem.

As Cassie signed this, her eyes darted around the room, and I wondered if she believed what she was saying.

‘Is your friend a man or a woman, Cassie?’

I don’t know, she said, but her face flushed and I thought she was lying. They send me presents and leave me notes.

‘But you said you’re not allowed to tell us who they are. So, you must know.’

She shook her head. I thought I knew, but now I’m confused. She bit her lip and looked down at the floor.

‘Tell us a bit more about your friend then, and the things they’ve done,’ Singh said.

Cassie looked puzzled. I’ve told you everything: texting students, pretending to be other people. My friend told me all about it.

‘What about filming Bradley and Courtney?’