I'm not saying there can't be a dress code, but I thought we were talking about uniforms. Nobody presents themselves (or very few do anyway) through uniforms unless actually required to.
There should be some variation in student uniforms, as different people have different color palettes, body types, etc. Students should be taught these things, because they're unlikely to learn them anywhere else.
It's in the best interest of schools to suppress subcultures, so that they don't distract from the learning environment. What kids want to do in their free time should be reserved for their free time.