Schools are being urged to introduce more female-friendly fitness activities such as Zumba classes and rollerblading because so many girls are opting out of exercise.
Research suggests the gap between the amount of exercise girls and boys do widens during their time at school.
A study for the Women's Sports and Fitness Foundation says more than half of girls are put off by PE classes.
The WSFF wants schools to make sports lessons more appealing to girls.
The research was conducted by Loughborough University, which found big differences in the attitudes of girls and boys towards doing sport.
Those differences were wider among older schoolchildren.
Falling exercise levels
Eight-year-olds did similar levels of activity: about 60% of those questioned, both girls and boys, said they did regular exercise - at least an hour, five days a week.
But among 14-year-old girls, that figure had halved - only 31% said they exercised regularly, compared with 50% of 14-year-old boys.
The research found most girls wanted to do more physical activity, but many were put off by PE classes.
Some said they did not like exercising in front of boys, and they were not confident about their sporting skills.
A number felt teachers paid too much attention to the girls who were best at sport.
Girls were also concerned about what their friends thought about exercise, and said getting sweaty was not feminine.
And many of those questioned said they did not think there were enough female sporting role models.
The WSFF is writing to schools offering advice on how to make school sports more attractive to girls.
WSFF said some PE lessons were "stuck in the 1950s jolly-hockey-sticks style of the past".
"It's simply unacceptable that the overwhelming majority of our young women are leaving school with dangerously low levels of physical activity," said WSFF chief executive Sue Tibbals.
"We can't afford to keep ignoring the evidence that school sport plays a key role in shaping attitudes to sports and fitness."
The Youth Sport Trust said schools needed to do more to address issues such as girls feeling body conscious or lacking confidence in their abilities.
"Schools that deliver PE well recognise these challenges and offer a wider variety of sports and physical activity that make girls feel included," said chairwoman Baroness Sue Campbell.
"We would like to see all schools take this approach."