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].) (01:35:40)
School of Rock (2003)
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Directed by: Richard Linklater
Starring: Jack Black, Joan Cusack, Mike White, Adam Pascal
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Dewey Finn: God of Rock, thank you for this chance to kick ass. We are your humble servants. Please give us the power to blow people's minds with our high voltage rock. In your name we pray. Amen.
Trivia: Miranda Cosgrove, who plays Summer Hathaway, actually has a good voice and plays guitar. She had to take a 45-minute class to learn how to sing badly for the scene where she sang "Memories."
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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.