Continuity mistake: At the end when it is revealed that Courtney killed Liz, we see several of the teenagers standing right in front of the stage. As Courtney walks through the crowd several of the teenagers have moved away from the stage and further back in the crowd. They couldn't have moved that fast through the rest of the crowd.
Continuity mistake: When Courtney, Fern and Foxy are in the cafeteria and they're all drinking diet Coke, Fern has her Coke in her hand near her face when Courtney names her Vylett, but when the shot changes to Fern's face, the Coke is on the table. When the shot changes to show all three, the Coke is in Fern's hand again.
Plot hole: The whole point of the deception is that Fern Mayo, the geek, turns overnight into Vylett. No one at school has a clue about her real identity till Marcia and Courtney put up posters. So this deception carries on for several weeks before the two girls expose it. There is no way this would work. A school will not simply accept a new student without bothering to register her, yet when Fern Mayo just doesn't turn up to school one day, and Vylett does indeed, none of the teachers find this strange, or bother enrolling "Vylett" officially in school.