On his first day of public high school, privileged preppie-dork Charlie Bartlett (Anton Yelchin) has the good sense not to roll up in the family limo. He does, however, make the mistake of wearing a tie and crested blazer, and when a snarling bully (Tyler Hilton) queries him in the bathroom on his sexual preferences, he carelessly responds: “Is that a rhetorical question?” The bully, a by-the-book type, duly gives Charlie a swirlie.

Before long, though, the bully has become Charlie’s right-hand man. Aided by his vast holdings of prescription drugs — bequeathed by Charlie’s array of gullible psychiatrists — this string bean in khakis metamorphoses into a kingpin capable of leading the student body against Principal Gardner’s (Robert Downey Jr.) nefarious plan to have security cameras installed throughout the school. Ultimately, though, John Poll’s rebellious-teen comedy falls well below the high bar set by recent genre hits “Juno” and “Superbad.” An anything-goes kookiness pervades the first half, but the film then takes a trite turn that only serves to highlight its unlikely premise, and you realize that the world of “Charlie Bartlett” was better aligned back when the protagonist was getting dunked in the toilet.

The casting of Charlie — a private-school expellee who is desperate to be popular yet doesn’t discriminate between jocks and losers — as the grinning beneficiary of the greatest high school success story ever told is fanciful to say the least. Charlie doesn’t just sell his schoolmates Prozac and Ritalin, he also acts as their shrink. Whether it’s out of respect to him or due to some stipulation in the terms of purchase, his clients — from a depressive loner (Mark Rendall) to a cheerleader (Megan Park) who worries that none of the football players who lust after her respect her — sit down and discuss their problems with him in a bathroom stall, and inevitably leave feeling better. Charlie is a carefree soul who, in puzzling early sequences, puts on campy accents and plays the piano with gusto. In most public high schools, these qualities would not work in his favor, but in the fictional Connecticut one dreamed up by the filmmakers, they are minor hindrances at worst, and even help him secure the object of his affection (Kat Dennings).

Cut from the same plaid as Holden Caulfield, Charlie is a soft-hearted rebel. But “Charlie Bartlett” insists too much on having it both ways. Here is a protagonist who charms the principal’s daughter with his good-natured innocence, then punches her dad in the face — then apologizes profusely. It seems hypocritical for the filmmakers to indict Charlie’s doting mother (Hope Davis) when they themselves are too fond of the boy for his own good. Charlie, who has a tendency toward impromptu public announcements, interrupts a party at one point to declare he has lost his virginity — and receives a roar of approval. In a clumsy shift, the film has by this point abandoned the idea of having fun with its lead character and fully devoted itself to cheering him on.

The Russian-born Mr. Yelchin has a manner all his own — an elastic physical quality (he launches his chin forward like Jim Carrey) and a voice that seems to be late in changing. It may or may not be his fault that his performance appears more tailored for the stage than the camera. Mr. Downey’s beleaguered principal, who serves as both foil and rival to the upstart protagonist much in the way Bill Murray’s disillusioned romantic did in “Rushmore,” is something of a mess. He is estranged from his cheating wife, drinks too much, and is stuck in a job for which he is ill-suited, but we don’t see enough of his grown-up side to understand that his failure is eating away at it.

Like Charlie’s mother, Principal Gardner is a single parent, and a less-than-successful one. (His objection to his daughter dating Charlie is portrayed as misguided, but the relationship — which includes a slow-motion make-out session best described as grody — is saccharine enough to arouse any responsible parent’s suspicions.) Charlie’s father is in prison for tax evasion, an absence not all that noteworthy in a film that depicts just about every teenager as a victim of neglectful adults. Unlike more perceptive teen comedies, “Charlie Bartlett” doesn’t consider the prospect that the reverse might also be true. “Juno,” for instance, had the nerve to suggest that a defiant adolescent might, in fact, have the support of two loving parents. But more important, it kept the laughs coming. Unfortunately, that’s not the case here.