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.
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.
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.
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.
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.
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.
Continuity mistake: When Annabelle is in the shower, the overhead videocam turns and we see its feed on the laptop. In this shot, as it pans the shower from left to right, the shelf is visible on the right wall, with nothing hanging from its hooks. However, a few shots later a girl removes the towel and robe that are hanging from those hooks.
Continuity mistake: At the start, when Annabelle and her father drive past the schools front gate, the trees are in full bloom, covered in white flowers. Yet in the next shot from behind the car, the trees are bare. Then when Flash drives up to the front gate, the flowers are back, but in following shots the flowers are gone again.