The dulcet tones of portable phones can be heard in playgrounds everywhere. Decca Aitkenhead reports
A story has been doing the rounds of public schools. A recalcitrant pupil is being upbraided by his housemaster when his mobile phone rings. With a gesture to the master to cease his scolding, the young boy coolly answers it. Following a brief conversation, he apologises to his teacher, explaining: "It was my stockbroker."

History does not relate the boy's fate. But the school imposed a blanket ban on mobile phones, and this embargo is now appearing in school rulebooks elsewhere. For teachers accustomed to catapults, this is a bizarre disciplinary development. For their charges, however, it is unremarkable. The mobile phone revolution has reached its logical target: the dial-obsessed, digit- happy, talkaholic teenage market.

An NOP survey carried out last month revealed that a third of the entire mobile phone market is now taken up by people aged 24 and under. Swatch, the maker of fun fashion watches, manufactures technicolour mobiles for funky young things rather than businessmen. Mobile phone culture is moving in on youth - the chatter of tiny voices means big business.

Jo Kernon, 17, has a mobile phone. "A good chat can last an hour. I can talk about anything - not only to make arrangements, just to chat." Her sister, Sophie, 19, says: "A call to a boyfriend is generally two hours long. I use the mobile phone not instead of seeing my friends, but as well as seeing them."

Youngsters have no problem finding things to talk about. The problem has always been finding something to talk down. For decades the telephone sat in drafty hallways, taunting wistful teenagers, a tantalising hotline to adulthood. Their parents regarded it with suspicion - an extravagance to be rationed, like Sunday outings in the family car. In the days before Buzby was a mere twinkling in an ad man's eye, it was definitely not Good to Talk.

Teenagers, however, grew less coy. By the late Seventies, joyriding on the family phone had become a modern rite of passage. No family home was complete without a phone and one hell of a row about it.

The arrival in the Eighties of the mobile phone looked unlikely to change anything. A bulky, unreliable business tool, its impact on young people rarely exceeded mild amusement on public transport - paunchy executives bellowing ineffectually into plastic boxes. Yet less than 10 years later, young people are sporting mobile phones like fashion accessories.

It was the introduction 18 months ago of the Mercury One-2-One phone that helped to trigger the change. One-2-One offers free local calls after 7pm and at weekends, transforming the mobile from an extravagance into a moneysaver. The service is now available to 30 per cent of the population, and for less than pounds 150 parents can buy their children a free phone service. More than a quarter of a million people have now subscribed.

Jo Kernon's parents bought her a mobile phone last year. "I didn't feel self-conscious. It made sense because it was cheaper. Most of my friends got one soon after."

Jo is studying for A-levels at a sixth-form college in north London. "I take it to school. So many people there have got them it really changes how you think about mobiles. When you see someone at school with one you don't think 'drug dealer' - you just think, 'normal person'.

"It's not embarrassing, and it's not a pose. My friend had her phone in an exam the other week, though, and she forgot to switch it off. Halfway through, it rang. She did feel a bit embarrassed by that."

That tortured eternal triangle - parent, teenager, telephone - may soon disappear from the adolescent experience. A Gallup survey carried out last year found that more than 50 per cent of parents issue a stream of threats to curtail calls, and more than two- thirds of children lie about their use of the phone. But if the cost of mobiles continues to fall, and the free service is extended to cover the entire population, the phone, one-time symbol of parental power, will become a feature of youthful freedom.

Some parents, of course, buy their children mobiles for peace of mind. But William Ostrom, of Cellnet, points out: "Parents, like everyone, look for permission to buy. Safety is as good a pretext as any."

Children have been quick to exploit the potential of such psychology; Madeleine Lewin, 16, says: "I wanted a mobile phone for safety reasons, not just for fashion." She got one.

Growing numbers of youngsters now see instant communication as a prerogative, not a privilege; the Gallup survey found that three-quarters of children rarely, if ever, think about the cost of calls before they pick up a phone.

"My 10-year-old daughter is totally uninhibited with my mobile in public," marvels William Ostrom, a spokesman for Cellnet. "We find that older users still prefer to buy the flip phones, because they hide their mouths in public. Younger users go for the flashier compact models. They just don't care."