Harry Thomas looks like he’s just witnessed a car accident. As he walks slowly into the staffroom I brace myself to catch him if he faints. “Are you OK, Harry? What’s wrong?” Staff exchange worried looks.

“It’s her,” he whispers. “She just came in and stared at me. Her eyes were hollow like a dead person’s. And she refused to leave the room.” Someone fetches a glass of water.

Harry Thomas, key stage 3 English co-ordinator, outstanding classroom practitioner and all round fit bit, is not easily shaken, but today’s Year 9 English lesson has got to him. This is alarming, as Harry works his classes like no one I’ve ever met. The kids love him, and his are the least disrupted lessons at St Brian’s. Some people call him the Horse Whisperer on account of his ability to pacify the wildest pupils. But today something’s gone wrong, and if he can’t cope, what hope is there for the rest of us?

“Who did this to you?” I ask. “Was it Estella Rampton? Michelle Moffat?”

But Harry isn’t listening. He continues in his monotone. “She was pointing her phone at me... threatening... saying she had evidence. Jesus, she’s a psycho.”

It must be Jade Crimmins, a particularly unpleasant girl who specialises in accusing male teachers of sexual assault. Her most recent complaint collapsed when someone noticed that her report of the alleged incident contained a reference to her “friend” Macaulay Culkin. It turned out she had lifted the whole thing from transcripts of the Michael Jackson trial.

I tell Harry that he must report the incident to her year leader. He turns to me, slack-jawed. “But Charity, she doesn’t have a year head, she’s a learning support assistant.”

Jessie McNally had been delighted when she was appointed lunchtime supervisor at her former school. At her interview she described St Brian’s as the only place where an adult had taken any real interest in her.

Unfortunately the adult in question was Nigel Horsmel, the deputy head, and the “place” was the behaviour unit, where she spent the majority of her schooldays. Now, thanks to workforce remodelling and Horsmel’s patronage, Jessie’s career is blooming. Some believe she could make the senior management team by September.

Suddenly, Horsmel’s voice booms over the staffroom PA, asking Mr Thomas to come to the office. Harry drops his glass, which shatters on the floor.

Horsmel is standing stiffly by the tropical fish tank. “Accusations have been made, Mr Thomas,” he barks. “Firstly, you neglected your teaching duties. Pupils were left to pursue their own interests.” As he speaks he holds up Jessie’s picture phone with a pixellated image of a group of well-behaved children reading a large thesaurus.

“Not only that but pupils gave presentations where they openly commented on their own work, and that of others. What the hell is going on, Mr Thomas?”

Harry is silent. “Assessment, Thomas! I want assessment! I want grades!”

Horsmel bangs on the desk. “For God’s sake, how do they know how they’re doing?”

“Because I tell them,” says Harry, composed again. “I talk to them, and they talk to me. I find it works very well, Mr Horsmel.”

Charity Casement is the alter ego of a north London teacher. Next week: Angel’s halo slips