Zero for conduct: "Bad Teacher"

There's something about Bad Teacher that doesn't work, which is a shame, as you can't fault its pedigree. The director is Jake Kasdan, whose Zero Effect and Orange County suggested an affinity with offbeam, character-driven comic material. The writers are Gene Stupnitsky and Lee Eisenberg, who've done as much as anybody to raise the U.S. version of The Office up to the high standard of its British equivalent. And the star is Cameron Diaz, here effectively reprising her standout role, the dissolute flake the actress played in 2005's In Her Shoes, within a promising-looking, R-rated School of Rock set-up.

Diaz plays Elizabeth Halsey, a self-centred golddigger first seen being dumped by her rich fiance, this scuppering her plans for a top-of-the-range boob job: David Paymer grabs the easiest day's work of his long and productive career in a one-scene bit as the doctor encouraging Diaz to fondle the breasts of the starlet drafted in to serve as the character's silicon calling-card. The script doesn't quite make the link between Elizabeth's golddigging and her day job - teaching at the ultra-lame John Adams Middle School - although there's one good gag about this educator's methods: hungover or otherwise ill-prepared, she elects to show her classes inspirational high-school movies (Stand and Deliver, Lean on Me, Dangerous Minds) in the place of any actual teaching. Only the new substitute teacher catches her eye: he's Scott Delacorte, a rich, ultra-conservative Christian played, for some reason, by Justin Timberlake.

Presumably a script conference or two got cancelled somewhere along the line, because these characters don't begin to make any sense - and it's difficult to laugh when you're expending more energy trying to figure them out than anybody working behind the camera has. To take the example of another snarky comedy with "Bad" in the title, we could see why Billy Bob Thornton had been reduced to the standing of a department store santa; that Jack Black, of all people, should fall into teaching (and that he should turn out to be good at it) was one of the pleasures of School of Rock.

Stupnitsky and Eisenberg can't come up with a good reason why someone as self-regarding as Elizabeth Halsey would have chosen to have got into teaching in the first place, nor how she's managed to sustain this career for any length of time - although perhaps disciplinary boards would be inclined to look favourably upon anyone with the body of the babe from The Mask. Delacorte, however, is a total question-mark: clueless even for a substitute teacher, a guy prepared to turn down flat the chance to get drunk with Cameron Diaz, it's possible the role fell to Timberlake because more experienced actors couldn't get their heads around this walking live-action cartoon.

To compensate, the crassness is amped up: yes, there's a diarrhoea gag, and Diaz troops through a carwash scene in short shorts, which'll doubtless look good in the trailer. There are, granted, smarter patches. I liked that its half-forgotten fetish song (a staple of these New American Comedies) was Shawn Mullins' "Lullaby"; Jason Segel is relaxed and funny as the gym teacher Halsey is destined to end up with (only in part because Scott Delacorte barely seems real); the writers provide a nice little showcase for the halting rhythms of The Office's Phyllis Smith as one of Diaz's colleagues; and Timberlake has a snickerworthy gag about Ethiopian cuisine. All in all, though, Bad Teacher is sketchy and skittish, skewing male in off-putting ways, and barely giving a hoot about the kids (as School of Rock most certainly did), except as foils for the star turn. As the upcoming Bridesmaids shows, if you are going to replace the lads that traditionally tear through these things with a ladette, you sure as hell have to give them more motivation than just a bad case of boob envy.

Bad Teacher is on general release.