I'm an American who attended public school, K-12. (I'm a senior in college now.) My city's school district passed an act when I was in middle school that mandated a dress code for all students--basically, uniforms that you didn't have to buy from the school. Schools individually set their own dress codes, but the baseline was business-casual attire in blue, black, white, and khaki. Specifics of color and permitted articles of clothing were left up to the schools.
My high school only allowed students to wear black or khaki dress pants or skirts (between knee and ankle length, pleated, no slits) and white or black button-down or polo shirts with sleeves no shorter than elbow-length. In winter, we were allowed to wear sweaters in black or grey. Shirts had to be tucked in. Shoes were to be black only. Boys could wear ties if they wanted, but only in the prescribed colors; no silly cartoon ties. Girls were not allowed to wear ties. Jewelry was limited to stud-type earrings, a single necklace, and one ring. Boys with pierced ears had to take earrings out for school. Makeup was allowed for girls, but it had to be subtle.
Needless to say, getting dressed every day was kind of a chore.
Summer wasn't too bad, we were allowed to wear knee-length shorts, but being a poor public school, there was no air conditioning or anything, and classrooms on the west and south sides of the buildings got really hot. We also had a lot of really strict rules about not wandering the halls, not eating in class (so no bottled drinks), and we weren't allowed to carry bookbags or purses, lest we smuggle in something illicit, like a pair of headphones or a book to read. :/
We did have "dress-down" days occasionally, usually one per semester. There were also some fund-raising things where you could pay five dollars or whatever and get a pass to dress down on a Friday.
They used to lock us into the cafeteria at lunch time. Half the bathroom stalls didn't even have doors (though I think that was because they just couldn't afford to fix anything.)
Weirdly, though, that school still had working showers in the locker rooms, and allowed students to change completely unsupervised. Only school I ever attended where that was the case.