I think school uniform can help a little with respect to blurring the boundaries of cliques and leveling the income differential. Can't level the playing field entirely, of course, but can go some way towards blurring it.

I do understand that dress is a way to express yourself... But that tends to be when you have sufficient funds to purchase that which you like to wear. For kids who aren't in the position... Wearing school uniform prevents your being ridiculed for not wearing the 'right' brand of jeans. Adults tend not to have their identity based around their dress in the same way that kids do when they are identifying strongly with 'emo' or 'hip hop' or whatever. People find ways to express themselves through gesture, verbal expression, art etc. I didn't like wearing uniform, don't get me wrong, but in hindsight I think it was a pretty good thing...