Continuity: When Terry Corrigan goes mad and teaches his students at gun point, he asks one of the thugs a question and holds his gun right up to his face. The tip of the gun is almost touching the thug's face, but from a different angle, the gun is further away.
Continuity: In the scene where Perry King puts the thugs arm on the table saw, the amount of blood, and it's position, changes, depending on the shot. This also happens throughout the rest of the film.
Revealing: At the end when Mr Norris starts to run through the hallways looking for the punks, he is ambushed and beaten up. If you look closely at his shirt, you can see a small pad underneath on his left, the exact spot where they keep punching him.