Shell Connections: Citizenship / Chatroom, National Theatre, Cottesloe, London

The opening double bill, on Wednesday, of the Shell Connections NT Festival was, to put it mildly, a pleasant surprise, featuring performances that were never less than competent and thoughtful, and were frequently stunning. The first play - one of the 10 commissioned annually for the festival - was Mark Ravenhill's Citizenship, a tragicomedy of sexual anxieties, performed by students from Glenthorne High School, Sutton (another production, by the National Youth Theatre, is on tomorrow night).

The central character, Tom, is burdened by a fear that he may be gay; but his attempts to clear up his confusion are one by one baffled. His friendship with Amy founders under peer pressure ("You ride her like your bitch?"). The sympathy he expects from his friend Gay Gary is not forthcoming ("That's just a name. You touch my arse, I'll kill you, see?"). The only gay teacher at school resists Tom's attempts to cast him as a mentor ("This isn't biology. I'm citizenship"). He even consults a ditzy tarot-reader, who, in the play's funniest scene, struggles to maintain an air of ethereal, New Age wisdom.

As in earlier plays such as Shopping and F***ing, Ravenhill displays an acute ear for the language of the moment - a satirist's grasp of how thinking can revolve around catchphrases and jargon. The plot itself is not up to much; the action consists, rather, in the ways that the characters shift between different registers - stoner slang, would-be gangsta, therapy-speak and the feelgood vapidity of modern public bureaucracy. One of the best lines comes when his teacher reassures Tom that his sexuality is not a problem: "You know school policy. We celebrate differences," he recites wearily.

But this up-to-the-minute feel can be a weakness - in all his work, Ravenhill shows signs of not recognising that the ways in which the present is different from the past aren't necessarily more important than the ways in which it is the same. Citizenship veers at times toward slick journalistic parody (and Ravenhill's plays always make me think what a brilliant journalist he would make: I don't know why some enterprising editor hasn't signed him up for the comment pages); but elsewhere it has a faintly worthy, issues-y feel - not unlike a school citizenship textbook. In the end, though Ravenhill dodges the temptation to draw lessons; that, together with his wit and the authenticity of the acting, made it an invigorating experience.

Preachiness never looms in Enda Walsh's Chatroom, set in a series of internet chatrooms, where a group of adolescents show off, flirt and play power games, all at a safe distance; though it seems that the distance may not be safe enough for the vulnerable Jim. Walsh's script begins with some superb comic riffs, delivered here with alternate viciousness and pathos by members of the Boomerang Theatre Company of Cork.

When things get serious, the pace flags drastically, and what ought to be a cathartic denouement has a muffled, uncertain impact. Still, the first half-hour of the script is unremittingly brilliant; and the way the cast responded to the script's ambiguities, hinting at the grim back-stories that underlie the comedy, was stunning in its sophistication and maturity. I'm cured: youth theatre doesn't scare me any more.

Season ends tomorrow (020-7452 3000)