Directed by Frank Launder and Sidney Gilliat to a script by Sidney and Leslie Gilliat, it was the fourth in a series of five St. Trinian's films. However it retained only George Cole, Richard Wattis and Eric Barker from the earlier films. Several new actors were brought in, including Frankie Howerd as Alfred (Alphonse) Askett, Reg Varney as Gilbert, and Dora Bryan as Amber Spottiswood, the headmistress. 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), and Cyril Chamberlain appeared as Maxie.
The first colour St. Trinian's film takes its inspiration from the notorious real-life mail train robbery of 1963, and is the fourth entry in the series based on Ronald Searle's cartoon schoolgirls.
"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 (Stratford Johns). He gives instructions to Askett about the robbery, Operation Windfall, using a variety of almost James Bond-like communications devices -- including a converted showerhead.
The crooks hide the loot in a deserted country mansion, and after waiting for the hue and cry to die down 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 to find out where the money is concealed. 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 obtain a large government grant for the school, thus allowing it to move to the mansion. This angers his staff who are normally Conservatives but early in the film are seen excitedly watching labour win as they believe St Trinians will be shut down. This aspect of the story was probably the reason why the Ministry of Education became the fictional "Ministry of Schools" for this film, to avoid possible action for defamation from a real Minister of Education.