At St Brian’s, it’s standard to lock classroom doors during lesson times; not to keep “them” in, but to keep the “the others” out. Our prospectus locates us at the heart of a thriving inner-city community. In reality, we’re ringed by high-rise flats, adult video shops and fast-food joints, giving the site more the feel of a badly defended military outpost than a school.

So this morning, when a man I don’t recognise passes me in the corridor, I challenge him. In line with the head’s guidelines, I say in a firm but non-confrontational voice: “Hello, this is St Brian’s. Are you a friend of the school’s?” The man smiles and says: “Well, I like to think so. I’m the head of key stage 3 English, a post I’ve held for the past three years. My name is Harry Thomas, and you’re Charity Casement, ex NQT and temporary head of history. Nice to meet you after all this time.” And he’s gone.

The fact that Harry knows me but I can’t place him is a little disconcerting. Where was he during my probationary year? And why have I never seen him in the 13 Horseshoes on a Friday night?

“Harry Thomas? Now there’s a strange one.” Brenda Gauche, PE teacher with responsibility for tittle-tattle, rolls her eyes. I’m in the staffroom waiting for Brenda’s killer fact about my new acquaintance, but she just gives a shrug. “The thing about Harry is... he’s just a teacher. Doesn’t drink, doesn’t chat up the Year 11 girls, doesn’t scheme against anyone, doesn’t... well, do anything much. Except teach. Bit of a weirdo, really.”

Then the conversation turns to meatier matters. Brenda lights up a Silk Cut and floats her latest theory: Judith Crock, my absent head of department, isn’t really off with stress - she’s carrying the secret love child of Les Twigg. John Baller makes as if to vomit. “Do you mind, Brenda, I’m trying to eat my Pot Noodles!” Someone points out that, according to her medical record, Judith’s had at least two hysterectomies, and Les is in his fifties. “Well, Des O’Connor‘s still turning out tadpoles and he’s 72,” says Brenda. “Anyway, what about Anna?” Anna Hatch, drama teacher and mother of four, has apparently gone home again with “nanny trouble”.

Brenda leans forward and whispers. “Let’s just say 19-year-old blonde Slovenian girls can be a bit of a handful, eh?” Brenda is about to embellish upon this domestic melodrama when Nigel Horsmel, the deputy head, arrives with a revised version of the cover list.

“Anna’s cried off,” he sniffs. “Nanny’s died or something. And you lot wonder where the budget goes. Bloody supply teachers for the baby brigade, that’s where!”

Someone tuts, and faces turn. It’s Harry Thomas, hidden away in a corner with his laptop. “It might be cheaper to run a creche, Mr Horsmel. It would minimise inconvenient disruptions such as dead au pairs and may even encourage experienced teachers to return to work after their maternity leave.”

Nigel looks at Harry but can’t hold his stare. He blinks and looks away. As the bell goes I watch Harry packing away his computer. Late twenties, full head of blue-back hair, denim shirt and beige chinos. Brenda sees me staring. “I wouldn’t bother with him, Charity.” “Why ever not?” I ask, indignantly.

“Because we don’t know anything about him, for God’s sake!”

“Exactly.”

Next week: Someone to watch over Angel