Pupils at the Condorcet high school in Limay, around 50 kilometres west of Paris, are not allowed to wear tracksuit bottoms to school.
But that didn't stop them on Tuesday from protesting against the dress code, with around 140 turning up to school wearing jogging bottoms (or sweatpants, as some call them).
There were another 400 people there in support, reported Le Parisien newspaper.
Not everyone at the school is against the ban, with one student telling the paper: "It's the rule, so we have to abide by it. Anyway, it's good preparation for the workplace."
One of the organizers responded: "Yeah, but we're not at work yet, so we have the right to enjoy ourselves."
Also on Tuesday, students at the François Couperin high school in Fontainebleau staged a protest of their own after they were told they couldn't wear ripped jeans to school.
Around 30 students gathered to protest the dress code, the majority of whom were wearing ripped jeans.
"A friend of mine was sent home in the afternoon for wearing ripped jeans," one student told Le Parisien.
"I don't wear them myself, but what a shocking attack on our freedom of dress. This is part of our personality, it's a public school."