Cynthia Thyme is a woman transformed. The one-time design and technology (or, to be truthful, needlework) teacher who shocked us all by leaving St Brian’s at the age of 64 to become a governess in Colombia, has returned.

But this is a very different Cynthia from the one who left last summer complaining that she didn’t understand kids any more (which, considering that she entered the profession some time just after the Suez Crisis, perhaps isn’t that surprising).

Former colleagues stand under a banner draped across the hall that says “Welcome home Cynthia!” clutching their polystyrene cups and digestive biscuits. Everyone wants to hear her story. All we know is that she secured a post with the family of an army colonel, but that after a few months her dream of a new life away from the mean corridors of St Brian’s went badly wrong when she was reported missing.

After this things become hazy. Some say she got caught up in a poncho yarn scam that went wrong; others that she was asked to look after some “class As” and, assuming they couldn’t be any worse than Class C back at St Brian’s, inadvertently volunteered to become a cocaine mule. Then there was the rumour that she had been kidnapped by Maoist guerrillas who needed someone to darn their uniforms.

John Baller has an even more sinister theory. He says the DT teacher shortage is so acute in our authority that the Teacher Training Agency paid the SAS to rescue Cynthia. “A covert operation in the South American jungle is a damn sight easier than advertising in The TES,” he says. “And cheaper, too.”

But something isn’t right. Cynthia doesn’t look like the victim of unspeakable acts of torture. I try to recall the thin, pale spectre who wandered the corridors unnoticed by staff and pupils alike, and who spent most of her lunchtimes on the phone to occupational health or the pensions agency. The figure before me bears no resemblance to that burnt-out shell.

Wearing a pair of camouflage combat trousers and a “Che Lives!” T-shirt, she is lean not frail, her once watery blue eyes now contrasting dramatically with her tanned skin.

Judith Crock, formerly Cynthia’s closest ally, is trying to explain the intricacies of her pet terrier’s latest operation, but Cynthia is having none of it. She spots an attractive young male textiles teacher whom she regales with her theories on curriculum reform. “The revolution is not an apple that falls when it is ripe,” she tells him. “First you must shake the tree!” She stamps her foot and I notice she is wearing hobnail boots.

On Monday morning the head announces that Cynthia is returning to St Brian’s as a member of the PSHE team, or “re-education officer” as she insists we call her. She will be 10C’s tutor and also take charge of enrichment activities. No one in their right mind would consider taking either of these posts, but Cynthia Thyme is on a mission.

As I wait for the kettle to boil I notice that the timetable has been amended in bright red ink. New activities include weapons training, disseminating propaganda, and “purging petit bourgeois elements”.

Welcome back, Cynthia.

Charity Casement is the alter ego of a north London teacher. Next week: The devil in Angel