Teenagers - they're all gun-toting hoodies, teenage mums and socially inept mobile phone addicts, right? Of course not, but Chatroom and Citizenship - two one-hour plays from the National Theatre - do little to challenge the stereotypes. Written for the Connections project, under which established playwrights produce scripts for young people to perform, Enda Walsh's Chatroom focuses on Jim (Steven Webb), a depressed teenager looking for advice and understanding in internet chatrooms.

It isn't long before he finds himself on Chiswick's Bloody Opinionated' forum where self-styled revolutionary William' (George Rainsford) and fickle, bitchy Eva' (Jade Williams) make it their project to drive him to suicide. The tension and claustophobia builds (or is intended to) as the audience is shown how the isolation and anonymity of cyberspace can bring out the very worst in people.

With its underlying theme of young savagery, the play seems to fancy itself as Lord of the Flies for the internet age. But the dialogue is so unfeasibly eloquent (no text speak or slang here) and the characters so sketchily two-dimensional (a 'maths geek' who fantasises about Stephen Hawking), it is wholly unbelievable and carries no real menace.

The actors, none of whom look young enough to play teenagers, all do their best with the script, but it just doesn't ring true. William is too verbose and melodramatic, something dictated by his lines and not Rainsford's characterisation, while Eva comes across as cartoonishly malicious. Moreover, the play doesn't say anything new. It will come as little suprise to anyone that the internet is as much a force for bad as good, or that teenagers can be frighteningly nasty. We are told as much every day in the media.

But there is more to being a teenager than either high angst or banal discussion of Britney Spears and if I were that age (and it's not so distant a memory) I would be rather depressed at being portrayed in such a tired, patronising manner.

Mark Ravenhill's Citizenship is definitely the more enjoyable of the two; fast-paced and snappy, it blitzes through one boy's exploration of his sexuality with verve and humour. Tom (Ashley Rolfe) is all at sea on the important topic of which sex he fancies most and embarks on a shambolic journey to find out, resulting in home ear-piercing, a fumbled attempt at snogging his stoner male friend and finally, a baby.

His confusion and lack of direction is made worse by the responses of those from whom he seeks advice."You know the school policy," says his uptight teacher (Richard Dempsey), when asked who he should discuss his feelings with, "We celebrate difference. You report bullies."

It's certainly good comedy value but again, the characters are about as convincing as those in Little Britain, whose own Vicky Pollard appears to have informed the female parts. Gay Martin (Akemnji Ndifornyen) is portrayed as mincing and limp-wristed and with his "Fun, sex, money" motto, there is no attempt to paint him as anything more than an out-of-date figure of fun.

The numerous school groups in the audience applauded rapturously, but I left feeling let down.