The interior of the Bush is a blackboard jungle where four foul-mouthed, dead-end Essex kids give the new young English teacher, Zoe, a rough ride and a first-hand lesson in how not to cope with moronic behaviour.

Can things really be this bad in secondary schools? And can you believe that the teacher, on trial for her job, would bring a blue prosthetic penis to a rowdy citizenship class, with a handful of flavoured condoms, to demonstrate safe sex before behaving drunkenly, and intimately, with one of the fifteen year-old pupils who turns up at her flat after a fight?

The answer, I’m afraid, is yes to these questions. John Donnelly's new play has the terrible odour of complete authenticity. I taught in one such school, briefly, many moons ago, and plays such as Barry Reckord’s Skyvers and Nigel Williams’s Class Enemy have exploited the shocking truth that the wittiest, quickest, and often the cleverest, pupils are languishing on the brink of boredom, “exclusion” and despair.

What to do? Who knows, but the tension and danger, the nastiness, the threat to authority, and the spectre of illicit sexuality, always guarantee good drama. Joanne Froggatt, the sweet-natured under-maid in Downton Abbey, makes a stunning return to the stage as Zoe, hardly older than her own pupils, fighting bravely for her dignity, evincing a sort of glum perkiness with her downturned little chin and physical deftness.

Charlotte Gwinner’s production boasts four more sensational young actors – where do they all come from?! — as the pupils: Joe Cole as the most insolent, Holli Dempsey as the most sexually adventurous and Kerron Darby as Daniel, the most sensitive, inevitably dubbed "gay boy". Daniel’s dad was a taxi-driver and his recital of a piece of "the knowledge" is a symbolic riff on a world elsewhere.

Zoe is torn between the riotous classroom and the predatory attentions of her “mentor” Maz (Christopher Simpson) and the cynical, close-to-retirement head teacher Harry (Andrew Woodall). Unusually in such a new dramatist, a zinger of a first act if followed by a really well plotted second, and the play impresses, finally, as a powerful argument for teaching: even against all odds, a spark of humanity may fire in the friction.