Some people never learn

You will be relieved to hear that the title 187 refers not to the running time of this well-intentioned drama, but to the Californian police code for a homicide. Trevor Garfield (Samuel L Jackson) is a Brooklyn science teacher who becomes acquainted with those three numbers when he flunks a student. We've all put drawing pins on the teacher's chair, but this particular disgruntled pupil expresses his annoyance in a manner that you couldn't dismiss as impetuous high-jinks - first by scrawling "187" all over his tutor's text-books, and then by stabbing him in the back.

When we flash-forward, to join Garfield after his recovery as he goes about his new life as a supply teacher in Los Angeles, the film swaps the mortuary blues and greys of its prologue for soothing golden sunlight. You won't see the line "From the director of Waterworld" anywhere near the posters for 187, but it's exactly that man, Kevin Reynolds, who does a fine job of forging a cinematic language for his hero's state of mind. The picture opens with some prosaic but kinetic images, as Garfield cycles to work across the Brooklyn Bridge, the Statue of Liberty appearing to be cowering rather than towering in the distance, flanked by cranes that sabotage its grandeur.

The menace of the Los Angeles locations isn't so implicit. Jarring with the holiday-brochure hues are Reynolds' stark, brooding portraits of the thugs whose casual profanities and sideways glances feed Garfield's paranoia. 187 may at times brush shoulders with its closest cousin, the anodyne Dangerous Minds, but at least the bullies in this film aren't romanticised; you can tell they'd do more than just steal your lunch money.

Garfield tries to get on with the business of Making Science Fun, but as the distorted soundtrack and claustrophobic compositions indicate, he has a long way to go before accepting that not every student is out to kill him. Equally frustrating is the principal's refusal to mediate - while Garfield fears that every student may be a potential attacker, the principal regards each of them as a lawsuit waiting to happen.

This situation of ethical stalemate is crisply presented by the writer, Scott Yagemann, who is significantly more successful in his attempts to portray a system in chronic disrepair than he is trying to channel anger through his characters without wheeling out the soapbox. Jackson is an actor whose cool, masterful manner can bring dignity to the most undeserved project, and for most of 187 he succeeds in conveying the frustration of an adventurous and intelligent man imprisoned by his own fears. Of course, that's not nearly as tragic as the sight of an adventurous and intelligent actor imprisoned by the demands of a conscientious screenwriter.

The script also asks too much of Reynolds - he responds with a decision that feels very much like throwing in the towel, allowing the struggling students to graduate in slow-motion in the final scene, a banal pay-off to a movie distinguished by its visual audacity. But there are some sparks of honesty and originality on the way, in the eager young pupil who is so delighted to have a handsome tutor encouraging her to study that she responds in the only way she knows how - by stripping naked and offering herself on the sofa. And it's nice to find that the grizzled pro, a staple part of any inner-city school drama, and played here with abrasive glee by John Heard, isn't all he appears to be.

It's interesting too that 187 is located in a twilight world somewhere between gritty authenticity and surreal fantasy. The scenario of an embittered teacher taking his revenge on those who have wronged him is rooted in reality, and yet when Garfield's first act of vigilantism turns out to involve a syringe, a bow and arrow, followed by an impromptu amputation, it makes Grange Hill look like the work of Ken Loach. This initially engaging tone, teetering on the brink of absurdity, escalates to the point where the final showdown accommodates a group of hoods imitating their favourite scene from The Deer Hunter. Last I heard, Russian roulette had yet to replace the drive-by shooting as the LA homeboys' execution of choice.

Cheryl Dunye's diverting first feature documents her own search to discover the truth about an enigmatic black actress from the 1940s known only as The Watermelon Woman. It's an excursion into American history, issues of cultural representation and the importance of icons. And if that doesn't whet your appetite, you might care to consider the fact that it has a lot going on upstairs and a nifty trick up its sleeve. Sadly, it intrigues only through its inventive manipulation of documentary techniques - which is to say that what the form promises, the content falls far short of delivering. The emotional value of cinema is nicely emphasised, with memories spinning off from various interviewees, particularly the director's charming mother, as they riff on memories. But with its coy romance and wacky shots of our heroine and her chum dancing, I found that it was all too ingratiating to make its political points with any force n

`187' is on general release from tomorrow; `The Watermelon Woman' is at the ICA, The Mall, London SW1, from tomorrow to Sun, then in rep