The St. Trinian's Girls burn down the school building and are, subsequently, put on trial at the Old Bailey in London, found guilty, and await sentencing the next day by Judge Slender (Raymond Huntley). This leads to rejoicing at the Ministry of Education, and in Barset, the school's village home, where Sergeant Ruby Gates (Joyce Grenfell) and Superintendent Samuel Kemp-Bird (Lloyd Lamble) can finally plan their marriage, which is predicated on the end of St. Trinian's. However, beautiful Sixth-Form pupil Rosalie Dawn (Julie Alexander) has been flirting with Slender during the trial, where she gives him her telephone numbers, and continues flirting at the sentencing session. Consequently, when Professor of Philosophy Canford (Cecil Parker) of the University of Baghdad suggests that, rather than punishment, the girls need sympathy, and explains that he has funds to buy a new school building, and with the help of noted educator Matilda Harker-Packer (Irene Handl), the girls can be rehabilitated, Judge Slender ignores the guilty verdict and gives him a year to accomplish his aims. This causes dismay at the Ministry, where Butters (Thorley Walters), on the advice of his psychiatrist, does a pastoral dance to calm his nerves. The revival of St. Trinian's also means the end of Gates' and Kemp-Bird's marriage plans. The girls, led by Prof. Canford, new Headmistress Harker-Packer, and the new teaching staff, move into the former Hannington Manor, now the new St. Trinian's school building.