Pupils have protested over ‘gender neutral’ uniform policy with police called to the school gates.
Priory School in Lewes forced all pupils to wear trousers in 2017 in a bid to be inclusive, following "concerns" over the length of skirts.
At the time, the school said only new students would be required to wear the new uniform.
But then shortly before the summer holidays the school stated that all students, not just newcomers, will be required to wear the "gender neutral" uniform.
But furious parents of year 11 pupils have said they will send their children into school wearing the old uniform anyway.
Children were met on Friday morning with two community service officers who aided the school, which turned away students wearing the old uniform.
One parent of a year 11 pupil, who did not want to be named, said: "This is not about the uniforms being gender neutral.
"This is about children and parents complaining about having to buy completely new uniforms for only a few terms.
"It is not a good situation especially as it is my daughter's final year of school and it's really disruptive."
The parent added: "The school made the decision to do this right at the end of term without any time left for us to sort something out - so it's a bit like Brexit."
Parents say they have been in discussions with the school about the possibility of a compromise.
A spokesman for the Priory School said: "Our uniform is designed to be a practical uniform which encourages students to be ready to focus on their school work and activities.
"Our uniform also helps us to dilute the status placed on expensive clothes or labels and challenge the belief that we are defined by what we wear.
"There are at least 40 other schools which have a similar uniform requirement.
"Our core purpose remains the quality of teaching and learning and we aim to achieve this by maximising the time spent on planning, delivering and evaluating the quality of provision."