At lunchtime the corridors of St Brian's become the property of the kids.
Like a subterranean tribe emerging from the shadows at nightfall, as the bell sounds the hordes pour forth and, for a terrifying 45 minutes, the school is theirs.
Teachers cower in the staffroom, spooning microwaved noodles into their mouths to a background of foul screams, shattering glass and the whiff of cannabis.
Stragglers who have returned late from lessons are easy to spot: they sit in the corners, muttering to themselves and letting out the occasional sob.
Sometimes they ask to borrow a tenner because they've lost their purse or wallet "out there". Standing between us and the barbarians without is the thin line known as the lunchtime supervisors.
When I was at school, we had dinner ladies, whom the children treated like surrogate grandparents. Sometimes they were your grandparents. These days the job spec is a bit different: supervisors have to be hard-nuts, ready to fight dirty and equipped with the gamut of legal restraint techniques. John Baller, the army veteran in the maths department, runs residential survival courses for prospective recruits.
Whatever John does it works, because these ladies mean business. Jessie rarely speaks; her facial tattoos and slack-jawed gum-chewing convey her message nicely. Then there's Charlie, the poacher turned gamekeeper: a former pupil who returned on the recommendation of her psychiatrist because she had become chronically institutionalised after three years in the behaviour unit. She knows all the tricks because she wrote the manual.
Hazel, the team leader, is probably the most terrifying of the three; with a vocal range rarely heard outside the London Philharmonic Choir, she specialises in disabling her foe with high-frequency sound. An unfortunate side-effect is a rather higher than average incidence of tinnitus among St Brian's pupils.
It's 12.20pm and I'm stranded in no-man's land. The 10 minutes I spent deleting the pornographic pop-ups from 10C's computers have made me late leaving for lunch. Suppressing my panic, I walk briskly across the playground, looking straight ahead. I try to ignore a pack of girls gathering to my right, but can't disregard the unmistakable sound of two Year 9 pupils, Cherelle Carson and Nadisha Harris, exchanging insults.
Cherelle is telling Nadisha she is a bitch for thieving her necklace.
Nadisha clutches a sparkly accessory that spells "Cherelle", claiming it's a present from her auntie's cousin's sister who also happens to be called Cherelle.
Suddenly the mob parts and Hazel towers before us, flanked by Charlie and Jessie. Hazel screams: "You see what I do! Go on, you just see what I do!"
None of the children waits around to find out, and I watch as they flee towards the main building, hands clasped over ears.
As I stroll back to the staffroom, the bell rings to signal the end of lunchtime and, like those towns that changed hands hundreds of times during medieval wars, St Brian's is once more in the possession of its staff.
Next week: Going insane at Inset. Charity Begins: adventures of an NQT, Charity Casement's diary of her first year at St Brian's, is available from TES Books, pound;2.99. Tel: 0870 444 8633 or visit the TESbookshop at www.tes.co.uk