And they are more subtle at hiding their antics than the boys, say the experts
A teenage girl is being throttled by peers in a school changing room. Pinned against a wall, she is warned to "lose her virginity or be killed".
This could be a scene in a film, but it is a real incident related by Professor Ken Reid, chair of the National Behaviour and Attendance Review (NBAR), as he spoke of rising bad girl behaviour in schools during the launch of the review body's report last month.
The panel found that binge-drinking, bullying and truancy are all on the increase among girls in Wales.
And experts say schools are finding it harder to deal with the more subtle classroom antics of young women, who are acknowledged to be better than boys at hiding their bad behaviour.
According to Susannah Eagle, senior research fellow at London South Bank University, catfights - which may include hair pulling and scratching - are nothing new. But she says girls are now more likely to be prosecuted if caught.
Linda James, founder of Welsh anti-bullying charity Bullies Out, said there had been a definite increase in girls' misbehaviour, but it is not always spotted by teachers.
"Boys are louder, so it's very easy for bullying by girls to slip past a teacher's radar," she said.
Girls still outperform boys academically in Wales, but attendance rates for those of secondary age are now worse than their male peers. And while fewer boys are being permanently excluded from school, the number of girls has increased slightly.
A study by Liverpool John Moores University earlier this year found girls were five times less likely than boys to be told off in class. They found teachers did not always spot rule-breaking girls, who "are more likely to pass notes under the desk".
In a survey by Bullies Out, 60 per cent of children who admitted to bullying were girls. Most said they did it because they had also been bullied.
But Mrs James believes girl-to-girl bullying has become stronger and nastier. She said personal appearance was often the catalyst.
"If you don't fit in, you don't have the right clothes or make-up, and you're over or underweight, watch out."
Mrs James said girls were under pressure from peers and the media to look like celebrities, whether the bleached and spray-tanned Wags (wives and girlfriends of famous footballers) or skinny models in magazines.
A rise in binge-drinking, glamorised by celebrities such as Amy Winehouse, was also seen as a growing concern in the NBAR report, which said two-for-one offers in supermarkets and lax identity checks have made alcohol cheap and easily available to underage drinkers.
Girls also find it easier to get into clubs and bars by wearing make-up and revealing clothing.
Natalie Savery, trainee manager of the charity Drugaid, said the rise of the binge-drinking culture was having a huge effect. "Young girls are going out and trying to drink the same as young men."
A UK study published in January by the Youth Justice Board found offences by girls aged between 10 and 17 had gone up 25 per cent. But the research team found growing evidence that schools were calling the police for fights that would once have been dealt with informally.
Writing for TES Cymru last month, Professor Ken Reid, chair of the NBAR and deputy vice-chancellor of Swansea Metropolitan University, said he was "particularly concerned with the rise of disaffected girls and links with bullying and cyber-bullying".
According to Professor Reid, in the 1970s, the ratio of truants was 20:1 in favour of boys.
However, the slight majority are now girls, with truancy occurring at an earlier age. The report says that girls are now just as likely to be badly behaved and skip lessons as their male peers.
Pupils questioned over their bad behaviour by NBAR researchers often said they felt rebellious because they had to learn subjects for the sake of it - modern foreign languages was the most cited. Most of those who truanted said they had been victims of bullying.
But teachers, particularly the newly qualified, are ill-equipped to handle troublesome pupils, according to the report.
It says there should be much more focus on training, particularly at primary level.