Broadly speaking, this is Richard Linklater's French Cancan--that is to say, a humanist's joyful exploration of the musical in which the actors' personalities resonate as much as the characters they play. Or maybe it's what Jean Renoir might have come up with if he'd remade Don't Knock the Rock and cast 12-year-olds as the musicians. Though this seems like a personal film, Linklater was hired to direct a cannily commercial script by Mike White, about a rock 'n' roll loser (Jack Black) who, fired from his job and his band, impersonates his wimpy substitute-teacher roommate (White) to land a teaching position at an upscale elementary school. This infantile character hasn't got a thought in his head except for rock music, but somehow he becomes a model teacher, and through stealth and sheer perseverance he turns his fifth-grade class into an inspired gang of rockers. The kids, all real musicians performing, are wonderful, and so is Black; Joan Cusack is both charming and funny as the principal. 108 min. Century 12 and CineArts 6, Chatham 14, City North 14, Crown Village 18, Ford City, Gardens 1-6, Gardens 7-13, Lawndale, Lincoln Village, Norridge, North Riverside, 600 N. Michigan, 62nd & Western.