Even with a dress code or uniform, kids will still find a way to customize what they're wearing. The only purpose a dress code serves is to keep underage girls from slutting it up and underage boys from getting beat up by a rival gang for accidentally wearing a pink shirt (Don't ask, it happened at my school, pink shirts were banned for boys). Other than that it's useless. Still useless in practice because the girls will still be sluts and the boys will still be stupid outside of school. And there will always be cliques and bullying and all the other lovely tragedies that comes with Middle and High School a dress code or uniform will never change that. But in the end, High School will always suck, so the sucky implementation of a dress code would just be another drop in the bucket. You raise a valid argument.. but I think the main goal of schools is to basically prevent shit from happening in schools. They don't really care about what happens outside of school unless the students are somehow caught in their uniforms. The world is messed-up place and you really can't stop everything bad (e.g. "cliques, bullying, and other lovely tragedies") from happening, but they're just *trying* to minimize these incidences within their system.
I've gone both ways (private and public). It's nice not seeing certain fashion disasters from occurring when everyone's in uniform.