She swallowed, then nodded. My friend knew they were sneaking around, so they got into their school email accounts, and sent them messages pretending to be from each other. They got them to meet in the art classroom, and set up the video camera to film them. Her face fell. But then Ms Villiers saw it, and she got into trouble for not telling the governors. I didn’t want that to happen.

‘Why did your friend tell you about all this?’

They just said we were friends, and they cared about me. They needed me to pretend I got messages from Joe, so the others would trust me, and that I should keep secrets for them because they did stuff for me.

‘What did they do for you?’

Got me things I wanted. Make-up, clothes, shoes. Sometimes I come back from lessons and I find new things in my bedroom. They even leave me cider sometimes, she signed with a mischievous grin. The new phones were from my friend, but I wasn’t allowed to tell the others. They said it was better to message that way than on the school computers.

Forest sat back and folded her arms. ‘Do you realise that your friend might be a murderer, Cassie? Do you realise they might be the person who killed Mr Wilkinson and Mr Achembe?’

The girl shook her head. No, I don’t think so. They wouldn’t do that. They’re kind. They’re the only person who cares about me. All the others think I’m stupid, but they don’t know. They don’t know what I can do. I can make any of them do anything. As she signed this last sentence, her eyes flashed dangerously, and it was obvious just how much she had enjoyed the power this secret knowledge had given her.

‘Cassie, you say this person cares about you, but where are they?’ Forest spread her arms wide. ‘Are they here, confirming that you’re telling the truth? Are they protecting you by confessing to what they’ve done, confessing to asking you to keep these secrets for you? No, Cassie. There’s nobody here, only you.’

Tears sprang up in Cassie’s eyes. I’m scared.

Singh leant forward and smiled at her. ‘We understand that, but it’s our job to protect you and keep you safe. But we don’t know who we need to keep you safe from.’

It started out with the presents, Cassie said. They all had little notes, saying they were from a friend. Then I got text messages from my friend, just asking how I was. I felt like I could tell them anything and they’d understand. They didn’t ask me to do anything for them for ages.

‘Why didn’t you tell us any of this before, Cassie?’