I’m generally not in favor of uniforms, but this argument really goes both ways: who purchases (potentially very expensive brand) clothes in a school setting where the expectation is that kids constantly wear nice, new clothes to school? Even assuming that bullying or mobbing based on clothes isn’t an issue, the cost to keep buying outfits could easily be higher than the cost of uniforms.
That said, I’ve known problematic settings only by proxy. At my school, nobody gave a fuck about what students were wearing, there was no dress code, and I would have absolutely hated being forced to wear a uniform.