The great St Trinian's school reunion

Annabelle Heath is a church steward from Shoreham-by-Sea in West Sussex. "I had to torture Jill Evans. That's me turning the mangle," she boasts proudly. She is talking about events long ago, when she played "Maudie the bookie" in 1954's The Belles of St Trinian's. "I was only about 11. I wasn't one of the ones with stockings and suspender belts." Heath – who appeared on screen under the name Annabelle Covey – describes her costume with just a touch of regret in her voice.

Fifty-seven years on, Heath has assembled a small army of St Trinian's old girls (her torture victim Jill Evans among them) to attend a special screening of the film at the Barbican next week. No, they won't be wearing those scruffy uniforms and oversized straw hats that used to be de rigueur among the school's younger pupils. Nor are they likely to be in stockings and suspender belts, the outfit of choice for St Trinian's sixth-form girls. But will they be well behaved? "I think there's a bit of St Trinian's in all of us," Heath states ambiguously. "Once a St Trinian, always a St Trinian."

The screening at the Barbican is one of a number of Old St Trinian's gatherings Heath has organised since the film was remade in 2007. The idea for bringing the hellcats of the fourth form back together first came to her when she attended the premiere of the new film. At the post-screening party, she met Colin Firth and Gemma Arterton, among others. She had also brought along photos of herself in the original movie.

She says: "They couldn't believe that we were still alive!" Around the same time, a newspaper ran a letter asking what had happened to the original St Trinian's girls. Several responded, writing in to tell their stories, Heath among them. She then managed to get hold of the contact details for her old co-stars – and it was very swiftly a case of old St Trinian's friends reunited.

In 2008, Heath summoned all the St Trinian's alumni she could to a special reception in Windsor. Last year, the tearaways were summoned to Northampton for a celebration of the music of Malcolm Arnold – who scored Belles. It was there she met composer Neil Brand, who put her in touch with the Barbican.

Few of the girls went on to have professional acting careers. "Most of us have got pretty average lives," Heath says. After St Trinian's, she worked as an extra for a few years. (Her other claim to fame is that, in her mid-20s, she was Diana Rigg's "driving double" in The Avengers.) She says she was "just too shy" to make it as an actress or dancer. Portsmouth-based Polly Baber (who went by the screen name Pauline Drewett) these days teaches pilates. "I was the airhead, really," she volunteers of her role as Celia. "I was the smallest, the youngest. I was the only one who never had holes in my tights. I always had beautiful tights. And I had a nice short gym slip. My hair always looked nice. It was backcombed with a little ribbon, but it wasn't messy with straw in it, like some of them."

She paints a surreal picture of life on set. During shooting, her character was taught by the fictional teachers (played by Renee Houston, Joyce Grenfell, Alastair Sim, Hermione Baddeley et al). Then, between shots, she was taught by real teachers. "My headmistress came with me and I would have lessons."

Heath is now the wrangler-in-chief for St Trinian's reunions, of which there are likely to be many more. "Any time anyone wants a team, they come to me because I've got all the details." The old girls aren't under any illusions that audiences are coming to see The Belles of St Trinian's 50 years on because of their acting skills. Says Heath: "I don't think I am ever going to win a Bafta."

The special reunion event for The Belles of St Trinian's is at the Barbican cinema, London, on 12 April.