Abbi Marper is too shy to speak above a whisper, but she wants to be a policewoman or a nurse. Her friend Becky Billing is studying to be a plumber. Charlotte Wilson, the most chatty of the group, is having a problem narrowing her options. "I want to be a firefighter, but I also want to be a paramedic and a midwife," she says. "The trouble is, there's just too much choice."

Slumped in the plastic chairs of a Sheffield community centre, shovelling fistfuls of free sweets from the coffee bar into their mouths, the group of girls are all members of Aim High, a dance troupe set up by Becky Billing and Charlotte Wilson's sister, 17-year-old Lauren, two years ago after they got in trouble with the police.

"Lauren and I got into a car with some blokes one night and ended up getting home really late, so we told our parents we'd been snatched off the street by strangers," Becky Billing half giggles, but flicks her hair over her face and refuses to look up. Before the girls knew it, their parents had called the police and a kidnap investigation had been set up.

When the shamefaced teenagers owned up, they were cautioned with wasting police time and asked why they had done it. "It was because we were bored," says Becky Billing, who is 17. "There's nothing for us to do outside of school. My mum had youth clubs, sports stuff and drama when she was young, but we've got nothing."

With the support of the police, the Commission for Youth Enterprise and a few local groups, Aim High grew quickly from six dancers to 55. It now holds two classes a week, for young people aged eight to 18, and recently performed to Sheffield's mayor in the town hall.

"It's completely changed me," says Lauren Wilson. "I'm not an idiot any more, for a start. I've got plans and stuff I want to do with my life."

The assumption nowadays is that girls' lives have dramatically improved in recent decades. After all, compared with previous generations they have undreamt-of opportunities in terms of freedom and educational achievement.

How, then, to explain recent studies that have caused a ­groundswell of concern among experts? For, far from seeing the world as their oyster, it is becoming increasingly clear that teenage girls are a stand-alone demographic in crisis – a group about which much is assumed but little is known.

The first study that caused experts to question the quality of girls' lives was published late last year: a highly credible look at the mental health of teenage girls in Scotland.

Helen Sweeting's 19-year-long study, published in the journal of Social Psychiatry and Psychiatric Epidemiology, concluded – to the surprise of academics, experts and politicians alike – that young girls were deeply depressed.

Sweeting's research, although geographically limited, was substantial. Three times – in 1987, 1999 and 2006 – she had returned to the same part of Scotland to question up to 3,000 teen­agers about their mental state.

Over the two decades, Sweeting found that, while the 15-year-old boys she spoke to had experienced a small increase in psychological distress, the number of girls of the same age reporting mental issues from mild anxiety to more serious symptoms had jumped sharply.

The 1999 results were alarming enough: the incidence of common mental disorders including anxiety, depression and panic attacks among girls had increased from 19% to 32% (the increase for boys was just 2% to 15%).

But the 2006 study revealed an even greater leap. Girls across all social strata were now reporting mental disorders at a rate of 44%. Over a third admitted "they felt constantly under strain". Those who "felt they could not overcome their difficulties" had more than doubled to 26%. The number who said they "thought of themselves as worthless" had trebled between 1987 and 2006 from 5% to 16%.

When Sweeting published her findings, some questioned whether her teenagers were an anomalous group, their mental states a sad but unrepresentative snapshot of a specific community. But a number of other studies, both in the UK and elsewhere, have come to similar conclusions. Last week government research into 42,073 children between the ages of 10 and 15 concluded that teenage girls were a vulnerable demographic, urgently in need of help.

Dr Alison Tedstone, who led the research, said the choices being made by teenage girls regarding diet, lifestyle and other health-related issues were so consistently damaging that they had become "a standalone group of the population" requiring immediate intervention.

Brought up on the tough, Lowedges housing estate on the south side of Sheffield, Becky Billing and Charlotte Wilson can talk endlessly about the alcohol-fuelled adventures they have had over the years, the fights they have started, the brushes with the police and the friends who have become pregnant.

"All the girls I knew at primary school were really nice and normal, then we go to secondary school and they all went a bit mad," says Charlotte Wilson, 15. "From 12 or 13 years old, most of the girls I know just talk about sex, alcohol, sex, drugs and sex again. It's like it's this big competition and it gets everyone pretty stressed."

Her experiences echo a vast study of the well-being of youngsters across 30 industrialised nations. The recent Organisation for Economic Co-operation and Development report ranked Britain's teenage girls as among the worst off for health, lifestyles and school standards relative to public spending levels.

The report found that "risky behaviour" among girls – described as a combination of drinking, smoking and teenage pregnancy – was more acute in Britain than in any of the other nations apart from Turkey and Mexico.

Teenage pregnancy is far higher in the UK than the average across the OECD's survey. The UK has the fourth highest teenage pregnancy rate after Mexico, Turkey and the United States. In Britain, more than 23 teenage girls per 1,000 gave birth in 2005.

So what's going on? Experts have said it is highly significant that this slew of research has coincided with two fundamental social upheavals: the period in which girls began to outperform boys academically, and the obsession with celebrity culture and the pressure on younger and younger girls to become sexualised.

It is hard for those brought up in the 1980s to understand the world in which young girls are now having to develop their sexual and social identities. The worst that young women of the previous generation had to contend with was a stolid, lingering patriarchy, a sniggering, Benny Hill-style of humour that was obviously already on its last legs, and Page 3 girls; a lewdness that today seems more quaint than offensive.

The sexual politics girls find themselves confronting today couldn't be more different. The sex industry has moved from the margin to the mainstream. Girls are besieged by images that glorify a pornographic view of women. There is a lap-dancing club in every town centre, six-year-old girls are bought fashion accessories adorned with the Playboy logo, Shakira writhes on all fours in a cage on MTV.

Last Thursday David Cameron said children – and young girls, in particular – were having their ­childhood stolen by a "growing, unnecessary and inappropriate ­commercialisation and sexualisation that is beginning far too young".

The Tory leader has threatened to ban advertisers who improperly target young children from bidding for government contracts for three years under a Conservative administration.

But according to Natasha Walter, author of the new book Living Dolls, The Return of Sexism, the sexualisation of the young has already wrought ­irreparable damage on a generation of young girls. In her previous book, The New Feminism, Walter claimed that feminism was fast becoming "part of the very air we breathe". Now, however, Walter admits that she was wrong. She failed, she says, to predict how powerful, how pervasive and how sly the backlash would be.

The air she imagined carrying the perfume of liberal ideals into homes, schools and workplaces has instead, she says, turned toxic. "Feminism's own language of empowerment has been turned against it," she writes. "The language of empowerment has been harnessed to confuse sexual liberation with sexual objectification. The impact has been insidious and profound."

Girls today are growing up in an atmosphere of unapologetic crudity. Stripping is widely cited as a method of empowerment. A student claiming to be from either Oxford or Cambridge University published her online sex diaries last week, claiming to be, "unapologetically and unquestionably, a closet nympho". Walter calls it the "era of New Promiscuity", where emotion-free sex is both expected and celebrated. We are, she writes, living in a culture defined by pornographic sensibilities, where young women are willing participants in online "games" like Assess My Breasts.

Last month a survey of teenage girls found that more than half would consider being "glamour" models – posing almost naked for men's magazines – and a third saw Jordan as a role model.

It is no coincidence, Walter suggests, that anorexia nervosa, the disorder of pathological self-starvation, is on the rise, with an 80% increase in hospital admissions among teenage girls over the last decade.

Susie Orbach, author of the seminal Fat is a Feminist Issue and, more recently, Bodies, about the western obsession with physical perfection, says things started to get worse for teenage girls about 15 years ago but that the problems had accelerated recently, particularly with regard to body issues.

"What we think of as 'normal' now in the way girls relate to their bodies would have been considered serious cause for concern 20 years ago," she said. "It's to do with the aspects of celebrity culture that are proffered to them – the post-Thatcherite notions of success and money as a fast track to happiness; the rapid growth of the beauty and style industries, which prey on teenage girls; the hypersexualisation of the culture; and the ambitions of the parents, who want their daughters to feel the world is their oyster."

The impact on girls struggling to comprehend both themselves and the world around them is not hard to predict. Who, after all, wouldn't feel confused and unhappy being raised in this brave new world that demands super-skinny, super-sexy and super-brainy all at the same time?

Eleanor Martin, a 17-year-old from Hampstead in north London talks of how her string of A and A* GCSE results didn't seem enough for her parents. "They just seemed to take my exam success for granted," she said. "It was like, 'Well, you're a girl, of course you're going to do well.' I feel like I have to do more than do well at school and be a nice person to please them, but I'm not sure what else I can do. I sometimes think it might be a good idea to go off the rails for a bit, just so they do appreciate me."

This hothouse of expectation has created what the American psychologist Jean Twenge describes as a "narcissism epidemic" which is, she believes, disproportionately affecting women. Twenge analysed data on 37,000 American teenagers gathered over almost 60 years. She found that, while in the 1950s only 12% of the youngsters agreed that "I am an important person", by the late 1980s it was 80%. In 1967 only 45% of students agreed that "being well-off is an important life goal". By 2004 the figure was 74%.

In recent years the growth has accelerated. In 1982, Twenge found, just 15% got high scores on a narcissism personality index. By 2006 it was 25%, an increase largely down to the results of the teenage girls who took part.

"The narcissist has huge expectations of themselves and their lives," said Twenge. "Typically, they make predictions about what they can achieve that are unrealistic, for example in terms of academic grades and employment. They seek fame and status, and the achievement of the latter leads to materialism – money enables the brand labels and lavish lifestyle that are status symbols."

It is, in short, the Paris Hilton syndrome spread across millions of lives. Most of all, of course, it affects teenage girls, such as Isabella Grant, a 14-year-old from Edinburgh. "I want to be a famous catwalk model, and I don't see why that's unrealistic," said the slim, blonde teenager. "Everyone tells me I'm really pretty and that, if I work hard enough, I can achieve whatever I want. I've got an agent already and have already had a few jobs. I am completely determined. Naomi Campbell has already given up the catwalk. They need new girls, and I'm going to be the best."

The narcissism of young women could just be a phase they will grow out of, admitted Twenge, but she is concerned that the evidence of narcissism is present throughout highly consumerist, individualistic societies – and women suffer disproportionately from the depression and anxiety linked to it.

Back in the Sheffield community centre, Becky Billing admits finding the competitive pressure of her female peers so noxious that she has chosen to opt out entirely. "I trust my mum and my sister, but apart from them all my best friends are boys," she admits. "I mean, I'm glad I'm a girl and everything, but I wouldn't want one as my best friend. They're all messed up and I just prefer not to be around that."

Might such overwhelming pressure to do better, look better and have more explain the recent reports of sharp rises in aggression among girls? While young women aged 16 to 24 still have the highest risk of becoming victims of aggressive crime in this country, recent studies have shown a significant rise in the numbers of girls turning to violence themselves.

Youth Justice Board figures for last year show that, while overall crime rates are falling, there is a 50% rise in violent crime committed by young women, with girls now responsible for around 21% of offences that reach the courts.

But the changing attitude of girls to violence appears to be having even more worrying knock-on effects. According to a UK-wide survey of 14- to 21-year-olds by Engender, the women's rights group, one in three girls (and one in two boys) think there are circumstances in which it could be acceptable to hit a woman or force her to have sex.

Even Abbi Marper, at age 11, happily admits to being as physically aggressive as the boys in her group. "You have to stand up for each other," she says, her voice rising above a whisper for the first time. "And you have to stand up for yourself. You have to show you're strong."

Charlotte Wilson, too, is keen to dispel any notion that, as a girl, she might hold back from physical violence. "Of course I've been in fights," she laughs. She pauses and looks over at her mother, Tracey, sitting alert but calm on the opposite side of the table. "Remember when I got arrested that time, mum?" she asked. "It was when I had the fight with that old woman. She must have been about 45. She told the police I had tied her to a fence with handcuffs and hit her with knuckledusters. They let me go with a caution after a few hours because that woman had made it all up. All I'd done was shove her about a bit and she'd done the same to me."

April Foulds left school a year ago at 16 with a single exam success, a basic level NVQ in childcare. Two years too young to earn money through real work or bene­fits, April is a NEET: a growing population group defined by not being in education, employment or training. "What am I going to do with my life?" she repeats, sulkily. "Dunno. Maybe I'll get pregnant and get loads of money off the government." Her friends squawk in protest. "Only joking," she adds, vaguely.

For all their brash, surface confidence, Charlotte Wilson, April Foulds, Abbi Marper and Becky Billing don't seem to feel liberated or empowered. Instead, as they gain confidence and open up, they inadvertently reveal low expectations and poor self-esteem.

The message is that for modern teenage girls the encouragement to do better, look better and have more has become almost unbearable. They need help and they need it urgently – not only for themselves but for the next generation, whose mothers they will be.