Other mistake: After the concert, when Dewey is telling Gordon how awesome the lights were, Eleni calls him Jack. The actor that plays Dewey is Jack Black. (This mistake is mentioned in the Kids Kommentary [sic].)
Continuity mistake: After the kid's music class and Jack Black yells "You." to Zack, you see the kids around him sit down twice between shots.
Continuity mistake: During the scene where the kids are saying their version of the Pledge of Allegiance, the camera zooms in on a blonde girl. From shot to shot, which hand she has over her heart changes.
Continuity mistake: When Billy gives Dewey the paper bag with his clothes in it, towards the end of the film, the way the bag is folded at the top changes between shots.
Factual error: At the "Battle of the Bands" scene, the kids arrive and have less than the length of one song to get ready to go on stage. (We see the band up on stage and hear the Battle of the Bands person tell the kids they're going on after that band.) Yet, when they get up on stage, they've got outfits, hair, makeup - including fancy braids and spikes - that just plain couldn't have been thrown together in five minutes.
Audio problem: After Dewey and Miss Mullins go to the bar, Dewey is heard singing and snapping his fingers as he drives. However, you can see through the windshield that both of his hands are on the steering wheel and he is not snapping his fingers.
Continuity mistake: During the scene at lunch when Dewey is telling the teachers a story, we see that the teacher who is sitting at the end of the table near Dewey is pretty close to him, but after Dewey tells the "gym teacher joke" we get a view of the table and the teacher that was nearest Dewey is farther away than before.
Revealing mistake: In the middle of the scene where it shows what the class does throughout the week, Dewey is shown playing a bass guitar with no strings.
Factual error: When Dewey is getting the class to jam on "Smoke on the Water" for the first time, he tells the girl bass player to play a G, but he shows the guitar player the chords to the song in the key of A. The chords do not match the bass line. A real guitar player who has played this song would not make this mistake.
Continuity mistake: When Dewey is writing his name on the board, he begins writing and then erases the end to leave "Mr. S". In the next shot that shows the board, it still says "Mr. S" but it is in different writing.
Continuity mistake: When Dewey is eating a sandwich from one of the girls, the Saran Wrap around the sandwich changes between shots.
Revealing mistake: When Dewey is telling the parents at Teacher's Night how awesome their kids are he says that Gordon is a genius. Gordon's mother kneels down and mouths the words "Did you?" in response to Dewey's line that continues after this shot in which he goes on to say, "He programmed an entire light show."