Set during September 1949, confusion reigns when St Swithin's Girls' School is accidentally billeted at Nutbourne College: a boys' school. The two heads, Wetherby Pond (Alastair Sim) and Muriel Whitchurch (Margaret Rutherford), try to cope with the ensuing chaos, as the children and staff attempt to live in the newly cramped conditions (it being impossible to share dormitories or other facilities), and seek to prevent the children taking advantage of their new opportunities.
Additional humour is derived from the departure of the Nutbourne College domestic staff and their hurried (and not very effective) replacement with the St Swithin's School Home Economics class.
The main comedy is derived from the fact that the parents of the St Swithins girls would consider it improper for their daughters to be exposed to the rough mix of boys in Pond's school, and from the consequent need to conceal the fact that the girls are now sharing a school that's full of boys. Pond is offended at the suggestion that his boys are not suitable company for the young ladies of St Swithin's, but he needs to appease Miss Whitchurch to salvage his chances of an appointment to a prestigious all-boys school for which he is in the running, and which depends on his ability to prevent his current post presenting the appearance of a beer garden.
Matters come to a head when a group of school governors, from the prestigious establishment to which Pond has applied to become the next headmaster, pay a visit at the same time as the parents of some of the St Swithin's girls. Frantic classroom changes are made, and hockey, lacrosse and rugby posts and nets are swapped about, as students and staff try to hide the unusual arrangement.
Two simultaneous tours of the school premises are arranged: one for the girls' parents, and a separate one for the governors, and never the twain must meet! The facade finally collapses when the parents become obsessed with seeing a girls' lacrosse match at the same time as one of the governors has been promised a rugby match.
The punchline is delivered – a clever swipe at post-war bureaucracy – when, weeks too late, a Ministry of Schools official arrives, to declare everything sorted out. "You're a co-educational school, I believe; well I've arranged for another co-educational school to replace St Swithin's next week... Oh, it appears they're ahead of schedule." At this point, several more coachloads of children and staff appear noisily, and utter chaos reigns.
Fade out on Alastair Sim and Margaret Rutherford, quietly discussing in which remote and unattractive corner of the British Empire they might best try to pick up the pieces of their respective careers, with her mentioning having a brother who "grows groundnuts in Tanganyika."Alastair Sim as Wetherby Pond
Margaret Rutherford as Miss Whitchurch
Guy Middleton as Victor Hyde-Brown
Joyce Grenfell as Miss Gossage
Edward Rigby as Rainbow
Muriel Aked as Miss Jezzard
John Bentley as Richard Tassell
Bernadette O'Farrell as Miss Harper
Richard Wattis as Arnold Billings
Gladys Henson as Mrs. Hampstead
John Turnbull as Conrad Matthews
Percy Walsh as Monsieur Joue
Arthur Howard as Anthony Ramsden
Harold Goodwin as Edwin
Laurence Naismith as Dr. Collet
Stringer Davis as Rev. Rich
Olwen Brookes as Mrs. Parry
Russell Waters as Mr. West
George Benson as Mr. Tripp
Angela Glynne as Barbara Colhoun
Keith Faulkner as Unsworth
George Cole as Junior Assistant Caretaker at Ministry of Education (uncredited)
The acting was much praised, in particular Joyce Grenfell as one of the teaching staff of St Swithin's, while Alastair Sim's portrayal of the kindly headmaster, Wetherby Pond, was seen as one of his strongest ever roles.
The film was successful on its release, being the fifth most popular film at the British box office for 1950. The Belles of St Trinian's made 1954 is another comedy about a girls' school at which chaos reigned, which was also produced by Launder and Gilliat and featured several members of the cast of The Happiest Days of Your Life, including Alastair Sim, Joyce Grenfell, George Cole, Richard Wattis and Guy Middleton, with Ronald Searle providing the cartoons for the titles.