Now the Equal Opportunities Commission (EOC) is urging schools that do not allow girls to wear trousers to change their policy.
The commission has taken up the case of a girl who wants to be able to wear trousers to school but is not allowed to do so.
The EOC has declined to give details of the school and of the case for legal reasons, but says schools should listen to the views of parents and pupils.
Julie Mellor, chair of the EOC said trousers could be a smart alternative to skirts as well as and practical.
"Many women wear trousers to work - including MP's, business women, barristers and myself," said Ms Mellor.
Sex discrimination
"It seems out of step for girls not to have the option of wearing trousers as an alternative to skirts as part of their school uniform.
"In fact, there is a strong argument that preventing girls from having the option of wearing trousers on the grounds of sex is actually unlawful sex discrimination as it gives girls 'less favourable treatment' simply because of their sex."
Ms Mellor said it was vital to encourage young people not to be limited by old-fashioned, stereotypical ideas on men and women's roles in society.
"Dress codes play an important part in how we see ourselves and it is important that school rules on uniform are not inconsistent with the realities of modern working life," she said.
Two years ago the EOC successfully supported a teenager in Gateshead, Tyne and Wear who challenged her school's ban on trousers for girls.
Jo Hale, then 14, forced Whickham comprehensive school to change its policy after the threat of legal action.
'Ridiculous'
Jo's mother, Professor Claire Hale, expressed her dismay that the EOC was, yet again, having to step in to help schoolgirls wanting to wear trousers.
"It's ridiculous that after all the time and effort the EOC put into our case two years ago they're having to do it again and again," said Professor Hale.
Outdated policies on uniform were just a means of control for teachers, she added.
"It's time for the Department for Education to put their foot down, it's nonsense."
But the Department for Education has distanced itself from the matter.
"It is for school governors to decide whether to allow girls to wear trousers as part of their school uniform but they must take into account their responsibilities under the Sex Discrimination Act 1975," a spokeswoman said.
"It would ultimately be for the courts to decide if this was a sex discrimination issue."