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.
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.
Continuity mistake: In the scene where Dewey tells the class that he is going to be singing and playing guitar, Alicia says something like "Why are you in the band?" and the shot is of her back with her right arm behind her chair, but when the shot changes so you see the front of her, her arm is in front of her chair on the desk. This happens several times.
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: In the classroom scene just prior to the naming of the band, Dewey is seated with his feet on his desk playing his guitar. The two girls approach him with the SOR T-shirt. From their perspective (front) there is nothing near Dewey's feet, but when the view switches to Dewey's perspective, the rabbit on the desk is now only an inch or so from his foot. In the wide shot the rabbit is nearly a foot away.
Continuity mistake: At the title sequence, after Dewey's solo, and just before he stage-dives, the camera cuts from him, to the lead guitarist, then back to him. In the first shot of him, he is just putting down his guitar. In the shot of the lead guitarist, the neck of the guitar is visible as if he was still holding and playing it, but when it cuts back to Dewey, the guitar has been put down, and Dewey has his shirt off.
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."
Other mistake: During the credits when the band is having class, Dewey starts singing, "Credits got to roll. Look at that name there. I do not know that guy." While he sings that he pretends to follow one of the rolling names with his finger. Only problem is he's going the wrong way. He's scrolling down, but the credits are scrolling up. (Though an explanation is offered on the commentary, this is still a mistake within the film.)
Audio problem: When Dewey starts playing his guitar towards the end (when he was leaning back in his chair with his feet on his desk, then leaned forward and stood up on the desk) we hear the drums starting to play. In the first shot of the class, the drummer is not doing anything except sitting behind the set, and there's still a beat.
Audio problem: In the final battle of the bands scene, when they play Zack's song, you can see a lot of amps on stage but there are only three guitars. There is no need for so many amps with only three guitars and such a small venue. Also if you look closely, the Peavey amp next to the keyboard is the only amp that has a cable connected to it. All the other amps have the guitar input on the front but you can see that none of them are connected to anything, neither are they switched on because you can't see any lights on the front of them.