Jack Black isn't an actor, he's a clown -- and a one-schitck clown at that.

Compare any two-minute clip of his new comedy "School of Rock" to any of his scenes from "Orange County," "Shallow Hal" or "Saving Silverman," and you'll see the exact same tongue-wagging and eye-bugging mugging, the exact same frenzied, finger-knotting gestures and roly-poly, off-balance dancing, the exact same eyebrow-stitching failed attempts at momentary sincerity, and the exact same set-devouring dialogue delivery.

"Read between the lines, baby! Read between the lines!" he whispers then screams, whispers then screams while giving a three-fingered flip-off to the musicians who have just kicked the embarrassing stage-hog out of their band in this movie's establishing scene.

On the verge of also being booted from the bedsheet-draped corner of his best friend's apartment where he's been crashing for ages, Black's freeloading oaf is desperate for cash. So when a snooty prep school calls looking for his buddy (Mike White, the movie's screenwriter), a highly favored substitute teacher, Black usurps his identity and begins teaching a dutiful 5th-grade class of Central Casting archetypes to loosen up and rock out.

The story is stale formula, right down to the battle-of-the-bands finale -- established as the be-all, end-all of Black's existence in clumsy first-act expository dialogue. After a few days of sleeping through the school day, much to the chagrin of his restless A-students, he discovers the kids all play instruments or sing (or at least have the gift for the fine art of being a roadie), and he begins molding them into the guitar-grinding, drum-soloing band that could make his moment-in-the-spotlight fantasies come true.

Despite such methodical scripting and a litany of common-sense story chasms -- the tightly-wound, gray-tweedy principal (an underutilized Joan Cusack) finds Black very fishy yet never checks up on his class -- "School of Rock" almost gets by on the light-hearted enthusiasm of director Richard Linklater (usually more experimental with films like "Waking Life") that shines through from behind the camera. The movie has a contagious, forgiving cheer-along spirit that will likely sustain those less averse to Jack Black's repetitive antics than I am. (In the interest of fairness, I'll tell you that this week's San Francisco preview audience and most other critics I've talked to enjoyed this movie.)

But even if the picture weren't so trite and even if the dubious moral of the story didn't seem to be "never grow up and never take responsibility for your actions," Jack Black still strikes me as a stubby, baby-gruff, blue-light-special Jim Carrey without the capacity for genuine humanity. While he may have a talent for enjoyable scene-stealing in supporting performances (see "High Fidelity"), that doesn't make him a movie star or give him the chops to carry a feature film -- no matter how much he might make some people laugh.