I can't find any clear evidence on school uniforms; the reports and data seems to be somewhat mixed. I think it's important to look at the data for these kind of things, as you can make "common sense arguments" that go both ways quite easily.

More importantly, bullying generally affects only a fairly small number of children, and has been around forever; it's not like bullying was invented in the last 15 years. Not that I want to trivialise the effects of bulling in any way, but I'm skeptical it can be the cause of a fairly wide shift in the general population (we're talking about 44% of kids here).