The head, Dr Scarlett, is giving his end of term speech. His audience is what he would describe as challenging. A mixture of the bored, the hyperactive and the hungover. And that’s just the staff. The kids are even worse, and the hall resembles a high security jail after the inmates have just discovered the new arrival on B Wing is a serial child abuser. They’re out for blood, their last chance to create mayhem with impunity until September, and the only things standing between them and the havoc they crave are Jessie McNally, the lunchtime supervisor, and Wendy from the referral unit. Two women alone against the mob.

The situation is complicated by the presence of dozens of non-pupils of all ages - cousins, nieces, parents, uncles - the result of Dr Scarlett’s rather idiosyncratic interpretation of the extended services legislation.

St Brian’s is now open from about 7am to whenever Roy, the caretaker, can get rid of the last stragglers. And our catchment seems to embrace just about anyone who fancies popping in.

Scarlett ploughs on, a whirling dirge of nonsense about enriching the school community, moving forward together, building stepping stones... Any one of the staff sitting around me could have given the speech, word for word, we’ve heard it so many times. And it’s not the only thing that’s familiar. As I look around I find it hard to believe I’ve been at St Brian’s for two years. John Baller, Cynthia Thyme, Les Twigg, Sandie McSniff: they all look exactly as they did when I walked into the staffroom all those months ago, the NQT with a mission to make a difference. And Nigel Horsmel, the deputy head who runs the school with a rod of iron and a heart to match, stands, as ever, sneering at the mediocrities whose lives he twists and breaks at will. Even the kids seem unchanged. The Gunner twins may have advanced from 9C to 10C, but they’re no less terrifying than when I first encountered them in a mosh pit of a history lesson on the Boer War.

But one thing is very different. Harry Thomas. After five terms of assorted stalkers, fantasists, confused gays and under-age chancers, I finally thought I’d struck gold. It took me a while to realise that there’s nothing odd about Harry, which is probably why everyone thinks he’s so weird. He’s just a good teacher with a normal hobby (he plays guitar in an indie band) who happens to be bloody amazing in bed. My bed.

But Harry is leaving St Brian’s.

As Scarlett drones in the background, Harry leans over and whispers in my ear. “You’ve got to get out of here Charity, it’s madness. Come with me to India.” I’m trying to ignore the tingle in my ear that makes me want to turn round and tear his shirt off, when I realise that the head has shut up and the kids are all staring out of the window.

Someone has set fire to my prefab, and I don’t know whether to laugh or cry. As I watch my classroom burning, Scarlett puts a sympathetic hand on my shoulder. “Don’t worry, Miss Casement,” he beams, “we’ll put it out.

We’re good at that sort of thing at St Brian’s. See you in September!”

Charity Casement is the alter ego of a north London secondary school teacher. Charity Begins: Adventures of an NQT, her diary of her first year at St Brian’s, is available from TES Books, pound;2.99. Tel: 0870 444 8633