Here’s a little vignette (it’s a true story): a 15-year-old girl attends London’s top girls’ school. Her less brainiac little sister goes to another league-topping school nearby, only marginally less exalted. Their mother is collecting Girl A from School A, and remarks out loud on how very thin everyone is, indeed much thinner than the girls at the rival establishment. Girl A, with a toss of her hair, says: “Yeah, we even do anorexia better than them.”

Extreme? Actually, no. Being faux-cynical, pouty and contrary has long been part of growing up, but there’s a distinction between making it your life’s work to annoy your parents and teachers and having serious mental health issues. It turns out that this line is being crossed by Britain’s teenage girls, especially “high-achieving” girls from comfortable backgrounds, in vast and alarming numbers.

In a highly credible recent study, “GHQ increases among Scottish 15-year-olds 1987–2006”, published in Social Psychiatry and Psychiatric Epidemiology, it has become apparent that young girls are officially the most depressed section of the population.

The number of 15-year-old girls experiencing psychological distress is rocketing; the study shows they are the most mentally ill group of people in the country, with 43% of them emotionally distressed and 27% suffering mental illness (severe depression or anxiety). What on earth is going on?

It's that time of year again: the countdown that will lead, in a few weeks, to the inevitable pictures of clever girls brandishing their exam results on the front pages of the newspapers. There’ll be the usual trills about how girls have, once again, wiped the floor with boys, about how they reign supreme when it comes to brains — the discrepancy in achievement kicking in at the age of five, we were told last week. There will be the usual photogenic gaggle, all long hair and longer limbs, waving their pieces of paper triumphantly.

Some of the girls will look, to the eyes of well-upholstered middle age, alarmingly skinny. Some will wear long sleeves, even though the sun is shining. Because now is also the time of year when parents spend proper amounts of time with their teenage daughters, see them in swimming costumes and think “God — I can see her ribs” or “What are those marks on her arms?”

Then we’ll all move on and the girls who worked so hard, for so long, to achieve their spectacular exam results will be dismissed. Because we all know girls are clever, that they work harder, that they do better than boys, blah, blah.

“It happened last year,” says Grace, now 17. “It was like, yeah, you’re a girl, so you did well. Obviously. What do you want, a medal?” It was as though suddenly the longed-for prize, the string of A*s, became meaningless overnight. “Media pressure,” she says. “For me, it’s not to do with seeing skinny girls in Vogue. It’s to do with the papers all saying exams are getting easier, that As are easy to get — expected, if you’re a girl. So you get them and no one’s very surprised or especially pleased-seeming. It doesn’t make you feel good. You should be really happy, but you’re not.”

The results may be stellar but the cost of achieving them is sky-high. Felicity, 21, is smiley and cheerful, stylishly dressed, not remotely the kind of person you would imagine has spent half her life crippled by insecurities and low self-esteem. She always wears long sleeves to hide the scars from years of self-harming — she started at 13: “I had a board in my room with all the days of the week in sections and I would write on all of them ‘Work harder, eat less’ every day.”

For her, cutting herself was “a coping mechanism. And almost an achievement in itself, because you could see the results”. She says: “There’s so much pressure to be the perfect student — sociable, outstanding, academic. If your sense of self is bound up in achievement, you need to keep achieving. Otherwise your sense of self is lost and you feel worthless. I remember being told by a teacher that unless we were going to university then we might as well drop out and go on to a management training scheme at Sainsbury’s.

“The media doesn’t help at all. They have these massive education supplements, always criticising the results, and every year you need to do better. The government want so many people to go to uni and so there are so many more applicants and it gets more and more competitive. I’m sure that even by GCSE, girls are aware of how crucial it is to do well at every stage, because otherwise they’ll never get a job at the end.”

Felicity’s story does not surprise Helen Sweeting, one of the three academics who interviewed a wide sample of 15-year-olds from the same place in Scotland in 1987, 1999 and 2006 for the study in Social Psychiatry. She has collected figures that are far scarier than any single tale of woe. (More than 2,000 mid-teens were quizzed for the 1999 survey, more than 3,000 for the 2006 one.)

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