Hienuri mentioned in a recent comment that in his country almost all students wear a uniform. Here in the U.S., uniforms are worn mostly in Catholic schools, some private schools, some charter schools, and a few public schools. I have never worn a uniform to school; neither have my kids.
The last time I saw a lot of kids in uniform is when we were visiting England, where school uniforms are standard for both state-run and private schools. The part that interests me is how girls are dressed for school. They wear neckties and blazers, which in every other context are only worn by boys and men. What's the message here? I think the uniform says that for the purpose of school girls are honorary boys. As we all know, a girl being dressed or treated as a boy is taking a step up, whereas a boy being treated or dressed as a girl is being humiliated. (Thanks, guys!)
It's partly a historical accident. School uniforms were originally designed for boys because only boys went to school. When schooling became more customary for girls, it was easier to just fold them into the existing structure. The school uniform tells us that male students are the norm, and female students are an afterthought. (Similarly, in the U.S. we have school sports teams that are called the "Lady X", where X is the school mascot. The message is that the male team is the norm, and the female team is an afterthought.)
While girls are dressed as honorary boys from the waist up, they wear skirts below. We can't have them wearing the pants! The skirt in the uniform would have just about killed me as a child, if I had to wear them; I hated anything girly. Thank God I grew up at a time of unisex children's clothing.
Schoolgirl uniforms usually have short skirts; this dates back to a time when short skirts for girls, matched by short pants for boys, were a signifier of childhood. (My father once mentioned his "first pair of long pants" as a big moment for him.) Today, little boys are no longer dressed in short pants, but the short skirt remains in the schoolgirls' uniform, where it is now wildly inappropriate. Catholic schoolgirls in our area routinely hike their skirts up to show as much leg as possible, and you can imagine the distracting effect this has on the boys. It would be more modest to put the girls in pants, which doesn't mean it'll happen any time soon.
Cross-cultural footnote: I'm using "pants" in the American sense, meaning "trousers", not in the British sense, meaning "underpants."