Uniforms are not particularly useful, IMO. (And I say this as someone who has left high school long behind me, LOL.)
The grade school / middle school I went to had uniforms and poor kids and "misfits" were still picked on. Ah yes, I remember one group of snotty girls who would decide who would be "it" in tag based on what possessions their family had, for example if Girl A had a VCR and Girl B didn't, then Girl B had to be "it."
The high school I went to didn't have uniforms and the kids were much nicer and didn't have a lot of cliqueness. Sure, you had jocks, goths, etc., but everyone got along together most of the time. It was a friendly place.
Mean kids will act mean no matter whether they're wearing uniforms or not and nice kids will act nice no matter what. If a mean kid can't pick on someone because of their clothes, they'll find some other reason to pick on them.