In a short story by Saki, "the Schartz-Metterklume Method" is a bogus system of teaching dreamt up by one Lady Carlotta (who is mistaken for a governess and goes with the flow). It involves making children understand history by acting it out themselves, which - when studying ancient Rome - proves extremely dangerous. (Think Sabine women.) Let me assure you that this is nothing compared to the dangers faced by the staff and, primarily, pupils of Ribblestrop school.

Andy Mulligan's first foray into children's fiction is a blast of fresh air. It's weird and wonderful and very hard to define. The action almost exclusively takes place in and around Ribblestrop Towers, left roofless following an arson attack the previous term. The school motto becomes "Life Is Dangerous" and mere attendance proves to be an excellent training ground for this undeniable truth.

I suspect that Disney won't be in any hurry to turn Ribblestrop into a family movie although, miraculously, it does have a real feel-good heart beating at its core. The pupils drink and use firearms. They walk on railway lines, one hides in a freezer, and there's even an incident involving a chainsaw and a power cable.

But this is far from being a 21st-century attempt at updating St Trinian's. The alcohol is a tot of rum each, supplied by the headmaster to make up for the lack of heating (and roof). The weapons include a gun given to Sanchez, a Colombian gangster's son, hiding out at the school following a botched kidnap attempt on him; and a flintlock pistol and crossbow used by Casper Vyner, whose mother leases the building to the school (while still living in the damp South Tower). The attempt to cut the power supply is part of a planned rescue mission.

Amazingly, in the midst of the occasional train crash, credit-card theft and mind-control experiment, most of the students are a force for good for most of the time. They care about the school and they care about each other, so the reader cares. They include Sam, his friend Ruskin and around a dozen overseas orphans. Then there's the wonderful Millie, the only girl at Ribblestrop, around whom much of the action revolves.

It's what's going on beneath their feet that they should worry about. Despicable experiments are being carried out down in some old second world war tunnels. By around page 280, really unpleasant things begin to happen. Be aware, this certainly is not a light comedy for all ages. A major character is seriously injured in an act of heroism and a bent copper becomes less and less of a comic character as the heat is turned up.

The staff are headed by the extraordinarily good-natured Dr Giles Norcross-Webb, who is very much of the old school of everyone-pulling-together and looking-on-the-bright-side. He sails through the action blissfully unaware of just how serious things are, bringing real charm to the proceedings. He's ably assisted by Captain Routon, who sees every crisis as a challenge to be compared to one in his colourful military past, and Professor Clarissa Worthington, who proves her worth in many ways. The villain of the piece is Miss Hazlitt. And there's so much more to the ghastly Hazlitt than meets the eye.

Ribblestrop is disgracefully dangerous high-octane fun of the highest order: an outrageous delight. It was pleasing to learn that - unlike Saki's Lady Carlotta - author Andy Mulligan is, or was, himself a genuine teacher. You have been warned.

 The first two of Philip Ardagh's Grubtown Tales will be published by Faber in May. To order Ribblestrop for £6.99 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardian.co.uk/bookshop