Other mistake: After Kelly and the twins climb out of the sewer tunnel and hear footsteps, the twins sit atop the steel grating pretending to cry while the rest hide below. All the walls and paintings behind the twins are the very same ones behind Kelly, Taylor and Andrea later, when they've finally reached their target painting, in a different area of the gallery. We can even see the square of wood planks covering the area of the steel grating seen earlier. (01:13:35 - 01:24:00)
Continuity mistake: When Flash is writing Theft next to no.6 on the board, Blackmail has appeared next to no.5 - Flash couldn't have written it on the board previously because when we see the board (before Theft is written on it), there is a blank space next to no.5.
Continuity mistake: When the girls are crawling back along the cable, there is a shot from just in front of Steven Fry which shows them behind him, when every other shot shows that the cable is over the audience.
Continuity mistake: When the bank manager hands Miss Fritton the foreclosure notice, she is wearing a dark floral blouse, a black patterned vest and small pearl earrings. Then, when Beverly gives Miss Fritton the "red" and "blue" pills, Miss Fritton is wearing a completely different outfit and large mabe pearl earrings. She reverts back to the original oufit/earrings when she speaks with Carnaby. (00:35:40 - 00:37:30)
Continuity mistake: When Flash and the girls are going through the 7 Laws of Crime, kidnapping is written on the board next to the no.4. When it comes back to the scene a little later, Kidnapping is no longer on the board.
Continuity mistake: When Flash is teaching "The 7 Laws of Crime," he writes "extortian" which is spelled incorrectly with an "a". Yet, when he says, "If you don't start using your noggins...," the word "extortion" is spelled properly with an "o", but then it reverts back to the incorrect spelling in following shots. (00:40:30 - 00:43:35)
Deliberate mistake: After the heist, when Taylor and Andrea are told they have only three minutes to the end of the show, there is an overhead shot of them on the zip line that is flipped. Note that Cheltenham and St. Trinian's have switched sides and the letters on the prop books behind them are backwards. (01:27:45)
Continuity mistake: On the school trip to the National Gallery, Kelly finds the paintings location on the far wall facing her, with marble trimmed doorways behind her within an open area, directly opposite the painting. The girls plan is based on this location. However, when Kelly, Taylor and Andrea reach the painting during the heist, entirely different walls with paintings are opposite the painting. (00:45:55 - 01:24:20)
Continuity mistake: When Flash gives a lesson on crime, he asks for ideas, and the girls raise their hands. In the shot from behind the girls, the first girl he points to for an answer sits at the very left (his right) of the class, and we also see Taylors back as her hand is raised, who sits towards the center of the class. However, in the next close-up its Taylor who answers with extortion. (00:40:20)