I noticed one thing about the "people have to buy their kids school clothes now" argument about cost of uniforms that is being either overlooked or people don't realize. There are a lot of families that are poor that get their school clothes from relatives as hand-me-downs or go to thrift stores or look for the bargain clothes. Like Aunt said, school uniforms or even just restrictive dress codes require certain clothes that aren't necessarily (and in general likely aren't) going to be found in those situations. My family got a box of clothes every year or so from my great aunt in Michigan because her kids were a few years older than us and that was a major source of clothes for us. My siblings got very few new clothes. Most of theirs were hand-me-downs from the older kids. Even now with my kids, I shop at thrift stores and look for things on discount racks when buying clothes for them. I have boys, but I've seen plenty of stuff for girls that would be respectable. However, it isn't likely that you would find uniform items on such discount racks. And it also isn't likely that I would get their clothes for so cheap, even if buying them from the school. Plus, I would still have to buy clothes for them to wear after school and on days off. It isn't like this cuts down on costs, like some believe. And it really wouldn't cut down on time to get them dressed either if you are teaching your children to actually put their clothes away or pull out what they are going to wear the night before.
Now, I think that the discipline/increased learning is a benefit, but I don't think it is for every district. I think some districts do just fine without the need for a uniform. A simple dress code should do just fine for most areas and most people. There are certain districts where uniforms or a highly restrictive dress code would likely be very beneficial and should likely be implemented. But also, I think it should definitely include some that do not cost the school.
I don't think there would be much in the way of hiding the "haves" and "have nots" though, at least not in the older grades for the first few years of its implementation because these students already know each other. That knowledge won't go away just because there are uniforms. Plus, there are always going to be ways to identify those who don't have all that much money.
(Oh, and as for the restrictive hair cuts and hair colors thing, I think that is completely wrong. That should be a student's choice. Unless it is truly a distraction, such as hair that blocks vision or has offensive words/symbols in it, it should be allowed.)