BAD Teacher is exactly the one-joke movie that you probably expect it to be, but there are enough variations and shadings of that one joke to sustain its brief running time — just barely.

Cameron Diaz plays … a bad teacher. She secretly sips airline-size booze bottles during class, doesn’t bother to learn her students’ names and figures that showing them movies about education like Stand and Deliver and Dangerous Minds is just as good as educating them herself. Because you see, she’s not teaching English at a suburban Chicago middle school for the deeply rewarding experience of shaping young minds. She just needs enough cash for a boob job, which she thinks will help her land a rich husband.

Like the far superior Bad Santa from 2003, the key source of laughs here is the subversion of an authority figure who’s supposed to be trustworthy, reliable, even honourable. And, like Billy Bob Thornton in that earlier film, Diaz just goes for it. Director Jake Kasdan (Walk Hard: The Dewey Cox Story) photographs her as if she were a vixen in a Whitesnake video, and Diaz revels in a role that lets her be brazenly sexy and inappropriately funny all at once.

Her character, Elizabeth Halsey, isn’t a mean or cruel person, per se; she’s definitely selfish and scheming, morally devoid and perpetually intoxicated. But sometimes, twisted little nuggets of kindness inadvertently pop out, as in her admonishment of a nerdy, lovesick student to get over the pretty, popular girl in class. She’ll never like him, Elizabeth tells him, figuring she’s doing him a favour by being honest with him now, at a young age.

And you know what? She’s got a point.

But while screenwriters Gene Stupnitsky and Lee Eisenberg (The Office) ultimately soften Elizabeth a bit, they remain mostly faithful to her true nature and never inflict some implausible change of heart on her. The pacing and the tone of Bad Teacher aren’t quite so consistent, though, which often leaves the film feeling like a series of amusing moments rather than a cohesive whole. Still, those individual moments often connect, and that has a lot to do with the strength of the supporting cast.

Jason Segel is hugely likable, as always, as the school’s laid-back gym teacher, the voice of reason who sees through Elizabeth and still wants to hang out with her anyway. Lucy Punch, as the perky and perfect teacher across the hall, takes what could have been an annoying, one-note role and fleshes out a character full of passive-aggressive insecurity. Phyllis Smith has some wonderfully honest, awkward moments as a shy teacher who’s dazzled by Elizabeth’s insubordination.

And then, of course, there’s Justin Timberlake, Diaz’s real-life ex-boyfriend. He plays a proper and preppy substitute who comes from old money but teaches because he truly believes in all that stuff about the children being our future. He may not get a chance to let loose with the kind of charisma he’s shown on Saturday Night Live or in movies like The Social Network, but it’s always a pleasure seeing him toy with his pop-star image.

Still, Bad Teacher leaves you wishing they’d all misbehaved just a little bit more.

Bad Teacher, a Columbia Pictures release, is rated R for sexual content, nudity, language and some drug use.