The Belles of St Trinians is a comedy film set in the fictional St Trinians School, released in 1954. It and its sequels were inspired by British cartoonist Ronald Searle. Directed by Frank Launder and written by him and Sidney Gilliat, it was the first of a series of five. Alastair Sim stars in a dual role as the headmistress Miss Millicent Fritton and her twin brother Clarence Fritton.
A new term opens at St Trinians School for Young Ladies, striking terror into the local residents and police. Headmistress Millicent Fritton (Alastair Sim) has an unorthodox teaching philosophy, letting her students run wild.
The school is heavily in debt. The teachers have not been paid for months, but they stay on, as no other school would touch them. To add to her woes, Miss Fritton is blackmailed by her brother Clarence (also played by Sim) into re-admitting his daughter, Arabella, who had previously been expelled for burning down the sports pavilion. There is but one ray of hope: the wealthy, westernised Sultan of Makyad (Eric Pohlmann) has decided to send one of his daughters, Fatima, to the school at the recommendation of his English secretary. Miss Fritton quickly takes charge of the girls one hundred pounds pocket money.
Appalled by the thought of another term of mayhem, Police Superintendent Kemp Bird (Lloyd Lamble) goes to see Ministry of Education official Manton Bassett (Richard Wattis), but he wants nothing to do with the school. Sometime in the past, Bassett sent his best inspector to investigate; when he did not return, Bassett dispatched another, only to have the same thing happen. Bird decides to plant an undercover officer, his girlfriend Sergeant Ruby Gates (Joyce Grenfell), as the schools new games mistress. She uncovers all sorts of illegal and dangerous activities, including a gin still in the chemistry laboratory. Flash Harry (George Cole), a former school employee, handles the sale of the bootleg gin, as well as the girls sports bets.
When the students discover that the Sultans racehorse, "Arab Boy", is entered for the upcoming Cheltenham Gold Cup race, they use Fatimas influence to watch the horse work out. Impressed by what they see, the Fourth Formers bet on Arab Boy to win. When Miss Fritton learns of it, she also reluctantly bets the schools remaining funds on the horse to save her beloved St. Trinians.
Her brother, however, has bet a very large sum on another horse. His daughter tells him about Arab Boys performance, then suggests she and her Sixth Form cohorts kidnap the horse. Clarence resists the idea, but in the end has no choice but to agree. The Fourth Form find out and steal the horse from the thieves, hiding Arab Boy at the school. The situation pits the Sixth and Fourth Forms against each other in a wild melee over the horse. Also getting embroiled are the school staff, visiting former pupils and Clarences men. In the end, the horse makes it to the race just in time and wins, enabling the headmistress to get the school out of debt.
Ronald Searle appeared in a cameo role as a visiting parent. Roger Delgado plays the Sultans aide. It was also the first film appearance for a 17-year-old Barbara Windsor.
The opening scenes of the girls returning to school were filmed at All Nations Christian College. This includes the entrance gate of Holycross Road and the outside shots of the school.
The film was the third most popular movie at the British box office in 1954.