Jack Black has rhythm, but not the rhythm of a roly-poly man. Everything about him has always seemed a little too quick -- the eyes that pop open, in excitement and panic, at the slightest provocation; the words tumbling out in a barrage of dude-speak. Even if you liked his performances in ''High Fidelity'' and ''Shallow Hal'' (personally, I wavered between enjoyment and annoyance), it was easy to think of him as a walking cartoon actor, all fake tantrums and ironic flippancy. The image of Jack Black as a tubby lightweight, however, falls by the wayside as you watch The School of Rock, in which he reaches deep inside his riffing, strutting, head-banging self to give the single most joyful performance I've seen all year. Black is still a happy geek in perpetual overdrive, only now he draws on his musical skill, and his hipster shamelessness, to deliver the acting equivalent of a perfect power chord crunched with a demon smile.
''The School of Rock'' was made by gifted veterans of the American indie scene, but it's still the most unlikely great movie of the year. Directed by Richard Linklater (''Dazed and Confused''), from a screenplay by Mike White (''Chuck & Buck''), the film looks, at a glance, like ''Kindergarten Cop'' crossed with an Adam Sandler reject. Black's Dewey Finn is a beery, slacker musician of the old school. He may be barely out of his 20s, but he's no whiny thrash punk. He is, instead, the sort of overly zealous lead guitarist who thinks he's the king of rock because he delivers caterwauling three-minute heavy metal solos followed by crashing stage dives. Fired from his band as a result of his egomaniacal grandstanding, Dewey borrows the identity of his roommate, a substitute teacher named Ned Schneebly (played by White), and fakes his way into his own gig as a substitute, where he's faced with a class full of precocious fifth graders who wouldn't know Led Zeppelin from the Goodyear blimp.
Dewey doesn't care about the job -- he just needs the paycheck to cover his back rent -- but he's got to do something for six hours a day. Upon learning that the kids take classical music, he decides to teach them the only thing he knows: how to play -- and love -- rock & roll. Right from the start, the collision between teacher as slovenly, aspiring rock god and uniformed students fixated on their gold stars is hilarious and weirdly resonant, since what's at stake is far bigger than Dewey's charade: It's whether he can make the first post-rock generation feel the noise. The kids know their Christina Aguilera and Puffy, even their Annie, but they've grown up in a world of technological authority; your average Deep Purple or AC/DC riff sounds as foreign to them as a druidic chant. When Dewey takes out his guitar and teaches one of the kids to play the snaky doom melody from Black Sabbath's ''Iron Man,'' we're seeing the juicy spectacle of vintage rock treated as if it were classical music -- which, in a sense, it now is.
Rock, as Dewey explains, isn't just sound; it's attitude, an expression of rebellion against the man. ''One rock concert,'' he declares, ''can change the world!'' This particular attitude is so out-of-date that it marks Dewey as a relic, his heart stuck, like a needle on vinyl, in the '60s and '70s. The beauty of it is, he's a relic who believes, and these kids are such straitlaced overachievers that yesterday's cornball antiestablishment message now comes full circle to meet them. Black plays Dewey as a second-rate musician who's really a world-class fan. He's an imitator of other musicians who has absorbed their licks, tics, poses, and vital essences, so that his fearful eyes come burningly alive when he escapes into his rockin'-out fantasy. He's an air guitarist of the soul, and Black makes his passion wildly infectious. When he teaches the serious-browed Zack (Joey Gaydos Jr.) how to hold his guitar in a power stance, bobbing his head up and down in mock ferocity, and then instructs him to ''raise your goblet of rrrrrock!,'' Dewey's jubilance in role-playing becomes the students', and ours as well.
''The School of Rock'' is so witty and touching because of its innocent perception that rock, if anything, now belongs in grade school -- that yesterday's thunder riffs are today's kids' stuff. Dewey, after all, is really just an overgrown kid himself. Joan Cusack is pinpoint funny as the uptight principal with a secret jones for Stevie Nicks, and the kids, notably the preppy drummer Freddy (Kevin Clark), the angelically business-minded Summer (Miranda Cosgrove), and Lawrence (Robert Tsai), the nerd keyboardist who learns to be cool, are priceless: Their adorableness isn't icky but a function of their quizzicality -- their embrace of rock as an alien form that becomes their own. When they finally get up to play at the Battle of the Bands, it's an ecstatic scene, yet you may also wipe away a tear as you realize that Dewey has become a great teacher after all, and that Jack Black, raising his goblet of rock, now rules.