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The first cartoon appeared in 1941, but shortly afterwards Searle had to fulfil his military service where he was captured at Singapore and spent the rest of the war as a prisoner of the Japanese. After the war, in 1946 he started making new cartoons about the girls, but the content was a lot darker in comparison with the previous years.
The school is the antithesis of the Enid Blyton or Angela Brazil-type posh girls' boarding school; its pupils are wicked and often well armed, and mayhem is rife. The schoolmistresses are also disreputable. Cartoons often showed dead bodies of girls who had been murdered with pitchforks or succumbed to violent team sports, sometimes with vultures circling; girls drank, gambled and smoked. It is reputed that the gymslip style of dress worn by the girls was closely modelled on the uniform of the school that Searle's daughter Kate attended, JAGS in Dulwich. The films implied that the girls were the daughters of gangsters, crooks, shady bookmakers and other low-lifes and the institution is often referred to as a "female borstal".
Searle's St. Trinian's was based on two independent girls' schools in Cambridge - Perse School for Girls and St Mary's School. Searle, growing up in Cambridge, saw the girls on their way to and from school on a regular basis and they were the original inspiration for the cartoons and the character. Testaments to this fact can be found in the Perse School for Girls' Archive area where there are several original St Trinian's books, given to the school by Ronald Searle. He also based the school partly on the former Cambridgeshire High School for Girls (now Long Road Sixth Form College). 
In the 1950s, a series of St Trinian's comedy films was made featuring well-known British actors including Alastair Sim (in drag as the headmistress, but also playing her brother), George Cole as spiv "Flash Harry", Joyce Grenfell as Sgt Ruby Gates, a beleaguered policewoman, and Richard Wattis and Eric Barker as the civil servants at the Ministry of Education for whom the school is a source of constant frustration and nervous breakdowns. Searle's cartoons appeared in the films' main title design.
In the films the school became embroiled in various shady enterprises, thanks mainly to Flash, and, as a result, was always threatened with closure by the Ministry. (In the last of the original four, this became the "Ministry of Schools", possibly because of fears of a libel action from a real Minister of Education.) The first four films form a chronological quartet, and were produced by Frank Launder and Sidney Gilliat. They had earlier produced The Happiest Days of Your Life (1950), a stylistically similar school comedy, starring Alastair Sim, Joyce Grenfell, George Cole, Richard Wattis, Guy Middleton and Bernadette O'Farrell, all of whom later appeared in the St Trinian's series, often playing similar characters.
St Trinian's is depicted as an unorthodox girls school where the younger girls wreak havoc and the older girls express their femininity overtly, turning their shapeless schoolgirl dress into something sexy and risqué by the standards of the times: skirts are short and show the tops of the dark stockings that the girls wear, and busts are emphasized by the cut of the tunic and shirt of the uniform. St Trinian's is often invoked in discussions about groups of schoolgirls running amok.
The St Trinian's girls themselves come in two categories: the Fourth Form, most closely resembling Searle's original drawings of ink-stained, ungovernable pranksters, and the much older Sixth Form sexually precocious to a degree that must have seemed especially alarming in 1954.
In the films, the Fourth Form includes a number of much younger girls who are the most ferocious of them all. It is something of a rule of thumb that the smaller a St Trinian's is, the more dangerous she is — especially when armed, most commonly with a lacrosse or hockey stick — though none of them can ever be considered harmless.
In the first two films, St Trinian's is presided over by the genial Miss Millicent Fritton (Sim in drag), whose philosophy is summed up as: "In other schools girls are sent out quite unprepared into a merciless world, but when our girls leave here, it is the merciless world which has to be prepared." Later there were other headmistresses, including Dora Bryan in The Great St Trinian's Train Robbery.
In December 2007, a new film, St Trinian's, was released. The cast included Rupert Everett, Colin Firth, Russell Brand, Lily Cole, Talulah Riley, Stephen Fry and Gemma Arterton. Reviews were mixed. A second new St. Trinians film was released in 2009.
The school's coat of arms was originally shown as a black skull-and-crossbones on a field of white. This was later changed to a white tau Cross (symbolizing the "T" in Trinian's) on a black field bordered white.
The school has no fixed motto but has had several suggested ones. The school's motto is depicted in the original movies from the 1950s and 1960s as In flagrante delicto ("Caught in the Act"). This can be seen on the trophy shelf, above the stairs in The Belles of St Trinian's (1954). The lyrics of the original theme song by Sidney Galliat (c.1954) implies that the school's motto is "Get your blow in first" (Semper debeatis percutis ictu primo). A poem in one of Searle's books called "St Trinian's Soccer Song" states the motto is Floreat St. Trinian's ("May St. Trinian's Bloom or Flourish"), a sly reference to the motto of Eton (Floreat Etona - "May Eton Flourish").
In the 2007 film, a new school song was written by Girls Aloud called Defenders of Anarchy:
The school fight song: