Plot

In order to save their bankrupt school, a group of troublesome girls stage a robbery with a group of geniuses on their backs

St Trinians proudly continues to represent the unacceptable face of British education. When the new Minister of Education announces he will personally sort the place out he doesn't realise either the enormity of the task or that the headmistress is an old flame. The school is anyway threatened with closure by their bank; with the staff clearly a waste of space the girls realise the responsibility to save the day falls on them. Perhaps ripping off the girl with the pearl earring (a painting by Johannes Vermeer) might be the way out?—J-26

St. Trinian's has the worst reputation of any girls' private school in Britain, that bad reputation extending from the facilities, to the administration, to the staff and faculty, to the students themselves. The student body has its separate cliques as does any school, the cliques having the common bond of being equally subversive, being aided in their subversiveness by a shady character named Flash Harry. The one student who doesn't fit in is straight-laced and meek Annabelle Fritton, a recent transfer from Cheltenham Ladies' College, where she butted heads with the stuck up bullies who led the student body, especially Verity Thwaites. Annabelle's paternal aunt is St. Trinian's spinster headmistress, Camilla Fritton. Annabelle does whatever she can to make her father, Carnaby Fritton, take her away to anywhere else but St. Trinian's. The new Minister of Education, Geoffrey Thwaites, who happens to be Verity's father, wants to clean up the school system by making an example of St. Trinian's. What Geoffrey does not know Miss Fritton an old flame of his, is the school's headmistress. St. Trinian's faces further problems when the bank will soon foreclose if they cannot make full payment on their £500,000 debt. This situation combined with finding out why her father sent her St. Trinian's leads to Annabelle finding her place for the first time in her life. The girls find out about the problems, and come to the realization that they would never have the same amount of freedom they have at St Trinians, at any other school and as such decide to work with Flash Harry to devise some plan to come up with the money to keep the school open. The plan will take all of their collective cunning to devise and pull off, which includes the not so easy task of making it into the finals of the national School Challenge, an academic competition between schools. They have other obstacles including Geoffrey and Verity, who continue to want to show up St. Trinian's and Annabelle respectively at any opportunity.—Huggo

St Trinian's is an anarchical school for uncontrollable girls in London, England who try to save the school from bankruptcy by stealing the painting 'Girl With A Pearl Earring'. The film starts with Annabel Fritton (Talulah Riley) being admitted to the school. Annabel meets Kelly (Gemma Arterton), the head mean girl of St Trinians. Kelly walks Annabel around the school and introduces each of the different cliques in the school - Posh totty, Chavs, Emos, Geeks and First years. Annabel has a rough first night in the school where her towel and dressing gown are taken from the shower and she is broadcast on the Internet by the other girls while running naked from the shower to the dormitory. She phones her dad (Rupert Everette) and demands to be taken away from the school.

Flash Harry pays a visit to the school and makes a deal with the girls for some lab made vodka.

A few days later, the school has a hockey match against Annabel's old school - their rivals Cheltenham Ladies College. Their captain is Verity Thwaites (Lucy Punch), the daughter of Geoffrey Thwaites (Colin Firth), the education minister. The girls give a hostile and violent welcome to their visitors and the match ends in a brawl between the two schools.

Later in the film, Annabel overhears her father talking with Mrs. Fritton about how he thinks Annabel doesn't fit in with the family and hurts her feelings. Later that day Mrs. Fritton talks to Annabel and makes her a part of the family and gets her to release her anger. The other girls at the school give Annabel a make-over and makes her part of the school. Meanwhile, Geoffrey Thwaites is inspecting the school and the bank is threatening the school with closure.

Kelly hears the talk about closure and tells the rest of the school, they then make a plan to save the school which involves getting to the final of "School Challenge" and stealing the painting 'the girl with a pearl earring'. Meanwhile, Annabel is talking to Flash Harry about acting as a gay German art dealer. He initially refuses but Kelly gets him to do the job (as he secretly has feelings for her and does not want to look stupid in front of her). The school team for school challenge is made up of the three of the Posh Tottys, Chelsea, Peaches and Chloe. They get to the final by cheating.

During the final Kelly, Taylor and Andrea foil the security for the painting that they're after and eventually retrieve it but on the way back the cable that they used to climb over the top of the audience of the final of school challenge snaps just after Taylor (Kathryn Drysdale) and Andrea (Paloma Faith) get over to the other side. Kelly gets stuck with no other way to get to other side again. Mrs. Fritton sees that Kelly needs help and goes to help, having to knock out Geoffrey Thwaites on the way. Meanwhile, Annabel takes care of Verity Thwaites.

The school successfully gets the painting and sells it to Carnaby Fritton who doesn't realise it is not the real one which has been returned under the cover of St Trinians finding it. They get a £50,000 reward for returning the painting and the cash from Carnaby Fritton they pay back the money owed to the bank and St Trinian's doesn't get closed.