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. (00:50:00)
Revealing mistake: When playing "School Of Rock" at the Battle Of The Bands, there is one point where Dewey is heard playing a chord before his hands make the chord position. It's only out by about half a second, but it can be heard when his fingers aren't on the fretboard.
Revealing mistake: When Dewey is telling the parents at Parents' 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."
Answer: It is never said but If I was to guess, they got it from the guy he was pretending to be (Ned's) information from the school office.