During the opening credits, as Jerry is driving to Franny's house, there is a shot of Franny picking up some text books off of a table. Upon close inspection, the ink stamp on the side of one of the books says "Ogden High School". This is the name of the school where the film was made, not where it takes place.
Jerry's injuries from the fight disappear when the police arrive.
Snow on the mountains in the background disappears quickly.
The security guard Buddy knock's out in the fight scene suddenly disappears.
The name tag on the principal's desk clearly spells his last name "O'Roark". But at the end credits, it is spelled "O'Rourke".
In the school store vandalism scene, Jerry is seen to be coated with foam when he inadvertently activates the fire extinguisher. That type of fire extinguisher contains pressurized water only.
The security guard would not have thrown away the note threatening Jerry. Plot wise, It was done to make sure the rest of the movie happened, but the note was concrete evidence of Jerry's "problem."
In final fight, as camera rises, an extra (glasses, red flannel shirt, looks way too old to be a student) looks up at camera on its crane.
There's a typo in the end credits, where "2cd Assistant Director" is written as "2cd 2cd Assistant Director".
At the final fight scene in the parking lot, the crowd surrounding them is heard cheering, though no one is actually shown doing so.
When Jerry throws up in the toilet and flushes, we hear it but the water in the toilet doesn't go down.
Right before Jerry pays Buddy to call off the fight, Buddy says "You're never gonna learn, are you?". However, it is clear by his lips that he actually said "You never fucking learn", and the line was dubbed over.
When the football player goes to beat Buddy up in the library (after Jerry had paid him to do so), he pokes Buddy's chest with his index finger. When his finger presses on Buddy's shirt you can see the outline of Buddy's mic/equip.
When Jerry places the fire extinguisher on the counter, later smashing the cash register; upon the close-up shot, the reflection of the camera and crew member can be clearly seen.
When Jerry is in class and the teacher is scrolling through the names of the students on her clipboard, she has nice, long, painted fingernails. In the close-up of her running her finger down the list of names, you can clearly tell it's a guy's finger with a short, non-painted fingernail.