Directed by Frank Launder and Sidney Gilliat with a script by Sidney and Leslie Gilliat, it was the last in the series of four films, one in 1980, and two in the 2000s, and the first of all 7 of them (so far) to be made in colour. It retained George Cole, Richard Wattis, Eric Barker, Michael Ripper and Raymond Huntley from the earlier films. Several new actors were brought in, including Frankie Howerd as Alfred (Alphonse) Askett, Reg Varney as Gilbert, Dora Bryan as headmistress Amber Spottiswood, and Stratford Johns as the Voice.
Although asked twice, Joyce Grenfell refused to appear again as Sergeant Ruby Gates; she was later reported to have said that she regretted appearing in the St Trinian's saga. Raymond Huntley appeared as the "Minister of Schools" (a fictional title), having appeared in a different role in the earlier films; and Cyril Chamberlain appeared as Maxie.
The extensive use of trick gadgets in Alphonse's hairdressing salon (used to communicate with the gang's mastermind) spoofs the secret gadgets used in the James Bond spy films of the Sixties, while the gimmick of having the mastermind conceal his identity throughout the film (he is never seen on screen) spoofs the character of Bond villain Ernst Stavro Blofeld, who was originally only an unseen voice.
"Alphonse" Askett (Frankie Howerd) is a hairdresser who is also the operational leader of a gang of crooks who are led behind the scenes by an invisible mastermind (voiced by Stratford Johns). He gives instructions to Askett about the robbery, Operation Windfall, using a variety of James Bond-like communications devices—including a converted showerhead.
The crooks hide the loot in Hamingwell Grange, a deserted country mansion, and after waiting for the hue and cry to die down they return to collect the numerous mailbags which contain £2.5 million (the same amount as in the real robbery). However, following a Labour Party election triumph, the house has been converted into a new home for St Trinian's School for Girls. The crooks decide to infiltrate the school by sending Askett's delinquent daughters, Lavinia and Marcia Mary, to St Trinian's as pupils, with instructions to case the joint to find a means of recovering the money, secretly, from its hiding place. The crooks' subsequent attempt to retrieve the mailbags on Parents' Day, disguised as caterers, results in a climactic train chase between the robbers and the girls.
A sub-plot is the affair between the headmistress of St Trinian's and the Minister, who uses his influence to corruptly obtain a large government grant for re-housing the school, following the latest fire, thus enabling it to move into the mansion. This angers his staff who are normally Conservatives but who, early in the film, are seen excitedly watching Labour win the election, as they believe St Trinians will be shut down by a Labour government. This aspect of the story was probably the reason why the Ministry of Education became the fictional "Ministry of Schools" in this film, to avoid a possible action for defamation by the real Minister of Education.
A gang, whose unseen boss (voiced by Stratford Johns) directs them via radio, steal £2.5 million from a mail train, hides the loot under the backstage area of the ballroom at a deserted country mansion, Hamingwell Grange, and then awaits instructions on when and how to collect the money.
Ministry of Education officials, although Conservatives, are happy when the Labour Party wins the election, as they are sure the Labour policy will be to close all the private schools, including St. Trinian's. However, the new Minister of Schools, Sir Horace Bradford (Raymond Huntley), whose lover, we soon find out, is St. Trinian's Headmistress Amber Spottiswood (Dora Bryan), shocks the officials including the Deputy Minister Manton Bassett (Richard Wattis) and Butters, Controller of Schools, Public, Females, Second Category, Southern (Peter Gilmore) by giving St. Trinian's £80,000 for a new building. Spottiswood uses the grant to purchase Hamingwell Grange and then goes about regrouping her staff: Deputy Headmistress Mabel Radnage (Barbara Couper); Maths Mistress Veronica Bledlow (Elspeth Duxbury); Games Mistress Magda O'Reilly (Maggie Rennie - married to actor Michael Rennie; French Mistress Albertine (Carole Ann Ford); Art Mistress Susie Naphill (Margaret Nolan); Music Mistress Drunken Dolly (Jean St. Clair); and Chairman of the Board of Directors Flash Harry Hackett (George Cole). At the new school, Harry sets up a betting establishment and Spottiswood's new school supplies arrive, which include slot machines, and erotic books for the new library.
In London, hairdresser and gang leader Alfred (Alphonse) Askett (Frankie Howerd) gets instructions from the boss to retrieve the loot that night and regroups his men: Chips (Larry Martyn) his hairdresser assistant; Willy the Jelly-Man (Norman Mitchell), a toastmaster; Leonard "Len the Ledger" Edwards (Desmond Walter-Ellis), a bank manager; Gilbert the Wheel (Reg Varney), a driving instructor; Big Jim (Arthur Mullard); Maxie (Cyril Chamberlain); and two others. They go to Hamingwell Grange and find it occupied by the St. Trinian's School, whose staff and girls drive them away. Alfred contacts the boss and reports this calamity. The boss tells Alfred to enroll his daughters, Lavinia (uncredited, but possibly Susan Jones) and Marcia Mary (Maureen Crombie) at St. Trinian's, in order to get information about the school schedule. Once there, they break into the Headmistresses' office, find out that a parent's day celebration will soon be held outdoors leaving the school empty, and radio their father with this information. The boss puts in a low bid, and wins the catering contract; the gang, posing as the catering staff, is to retrieve the money after lunch, while everyone is attending the entertainment program outdoors.
Days before the celebration Rose (uncredited), while raiding the pantry, sees the school dog emerge from a hole in the wall with a wad of money; she goes through the hole, finds the stolen money and takes some. The next day at Harry’s betting parlour, where he is assisted by Georgina (Portland Mason, James Mason’s daughter - 1st on left) and an unnamed girl (Ingrid Boulting – Roy Boulting’s stepdaughter – far left) she places a £50 bet and he becomes suspicious about the source of her funds. In London, Bassett and Butters tell the Minister that the school supplies noted on the bills as nature books and laboratory supplies are, in fact, very sexy books and slot machines. The Minister dismisses their claims leaving Bassett and Butters dismayed. Eric the Liftman (Michael Ripper) suggests they provide evidence that there is only partying going on at St. Trinian's, by taking pictures. Bassett and Butters agree, and Eric goes along as photographer. That night, when Rose goes to raid the pantry, Harry follows her, sees the bags of money, and connects them with the train robbery and the £10,000 reward for the money’s return. The Ministry men arrive and take a picture of the sixth form girls partying, then go to the Headmistress to tell her of the impending problem she faces. When they knock at her door the Minister answers and tries to explain away his presence there by saying he has come to the celebration early in order to investigate their claims. When Spottiswood asks him to come back to her, the men realize she is the Minister's concubine, and foresee the end of his career. The Minister is worried about a scandal, so Spottiswood has the girls kidnap the Ministry trio. In London, the boss contacts Alfred with the escape plan via truck, train, and boat.
On celebration day the Minister is still worried. Spottiswood tells him to stop worrying: she will have the sixth form girls take some suggestive pictures with the Ministry men, and agrees when he says that would be blackmail. Harry, with the intention of demanding more than the offered reward, calls Edward Noakes (Colin Gordon), the insurance assessor for the robbery, claims to know where the loot is, and offers to meet Noakes. The Parkmoor Catering Co., staffed by the gang, arrives at St. Trinian's and, as a decoy, set up the catering end of their mission. Harry, disguised, meets Noakes, but someone recognizes him, and Noakes leaves. At St. Trinian's Alfred, using a Polaroid camera that the boss has converted into a two-way radio is told that Noakes has been sighted nearby talking with an unknown man, and must be eliminated if he comes to the school. The boss sends Alfred a photo of Noakes for identification. Harry phones the police to claim the reward, but his claim is ignored. At the lunch, one of the girls recognizes Gilbert as someone who used to be at the pub with her father, but thinks no further about this. On the dais, the Minister hears some rumbling under the stage, which, Spottiswood explains, are the three Ministry men blundering around in the cellar. After lunch, as everyone goes outdoors, Alfred finds that some girl has taken his disguised radio transmitter. Backstage, the gang pries open the floorboards and begin to retrieve the loot. Outdoors, it soon begins to rain and everyone goes inside again. As the Morris Dancing (The Westminster Morris Men) takes place on stage, the gang continues to retrieve the loot. When the lead dancer notices the gang backstage, Alfred takes his place until the gang has finished retrieving the money, and they all leave in the catering truck. Meanwhile, three girls bring Alfred's radio apparatus to Harry and turn it on. The boss, thinking Alfred has contacted him, explains that the gang must eliminate the unknown man who had been with Noakes, and transmits a photo of Harry. When one of the girls tells Harry that she had recognized one of the waiters, Harry realizes the caterers are the thieves. Some of the girls go to the ballroom to catch the gang, and one of them tells Spottiswood about the disguised gang. Harry then mobilizes the girls to chase and capture them. Now Noakes calls the police and tells them the stolen money has just left St. Trinian’s. Some police head for school, and others to the local train stations. When Harry finds the train with the loot getting away, he goes to the signal box and throws a lever engaging a stop sign; while the train is stopped two girls unhitch the van with the loot. When Harry disengages the stop sign, the crooks take off, use an extra lever they brought along to switch onto the line which will take them to the boat, disengage the switch, and hide the lever by throwing it in a nearby waterhole. Then they realize the van is missing, and are forced to waste time finding the lever to throw the switch, while a large group of girls, who have taken over a train and followed the gang, annex the van, and head back to the station. Finally, the gang throws the switch and chases the girls. At a different station, the police commandeer a train and, thinking they are behind the two trains, take off in pursuit, only to realize they are in front of the two trains, and reverse direction, as do the other trains. Eventually, the girls shunt the gang, followed by the police, into a station where they are blocked by a stopped train, and the police capture the gang except for Alfred. Meanwhile, the girls have switched to a different track and take off in the opposite direction and stop at Pudham, where they are found by other policemen and credited with returning the loot and earning the reward.
The next day, the newspaper reports 300 St. Trinian’s girls have been awarded MBEs, causing thousands to relinquish their rewards in protest.
The film was among the 15 most popular movies at the British box office in 1966.