usually only in private schools - but some public schools moved to uniforms a few years ago for a few different reasons:
1) so poor kids are indistinguishable from rich kids
2) supposedly kids in uniform are less likely to fight
and, in older years,
3) to keep guys and girls from wearing provocative clothes

I had to wear uniforms in public schools since I was in the 4th grade back in 93-94. So its been uniformed for awhile. At least here on the west coast.