About 150 parents and pupils have staged a protest outside a secondary school over gender neutral uniforms.
Priory School in Lewes, East Sussex, made trousers compulsory for new and existing students for the new term.
The school said "concerns" had been raised over the length of girls' skirts and new rules also catered for a handful of transgender pupils.
Protesters have said pupils should have a choice to wear skirts, while others believe clothes are being wasted.
Former Priory student and TV presenter Piers Morgan tweeted his support for the protesters saying the "gender neutral craze" was out of control and girls should be girls, and boys should be boys.
The Conservative MP for Lewes, Maria Caulfield, also tweeted: "Very disturbed to see the school turning away girls from Priory school because they choose to wear a skirt and calling the police on them.
"This is not how we should be treating the young women of Lewes."
Libby Murray, who is in her final year, said the new rule meant clothes were going to be thrown away, which would contribute to the climate change crisis.
She also said removing the choice for pupils to wear skirts because some wear them too short was "unfair".
"Girls roll up their skirts but that can be solved by better policing of it."
She added: "To make it gender neutral they have to let everyone wear skirts or trousers and have that choice."
In 2017, the school introduced a trouser-only policy for new students. It brought in the blanket ban on skirts for all students on Friday.
In a statement, it said students not conforming to the new rule would be asked to return home and change before being allowed into the building.
Pupil Nina Cullen wore a skirt to school and was refused entry.
"I haven't bought the new uniform and I don't see the point in wasting money," she said.
During extremely hot weather pupils had previously been allowed to wear PE shorts or skorts - shorts made to look like skirts.
However, a letter sent to parents in June said the decision had "created more problems than we wished".
It said pupils not following the new rule was "detracting" staff from teaching.