This Richard Linklater/Mike White project turns on a can't-miss idea: What if a $15,000-per-year prep school were invaded by a wild-eyed failed rocker, who proceeded to school the kids in the ways of the almighty power chord? Greasy-haired lunchbox Jack Black plays Dewey Finn, a shiftless but perpetually optimistic muso who gets bumped from his own band and has to bluff his way into a substitute-teaching gig to pay the rent. The half-hour it takes to establish Dewey as a chronic layabout and classroom fraud is merely OK slacker comedy, but the movie truly comes alive when the Dewster hits on the idea of enlisting the student body as his new backing group. Every riff-honing session that follows is a straight-up riot, thanks to Black's earnest histrionics and the kids' uniform sweetness. (Though they were cast for their instrumental abilities, their performances rank with anybody's.) While the cradle rawks, costar Joan Cusack brings funny poignancy to the part of the school's ice-queen principal, a hated disciplinarian who would secretly rather be Stevie Nicks. A hoot.