The high school my children are expected to go to currently has a draconian uniform policy- blazers must be worn, outside coats taken off once on school premises so that uniform is visible, an exhaustive list of requirements both for regular uniform and PE kit.
My nephew got 8 standard grades at 1, and 5 highers at A. He received no academic encouragement or praise, but was frequently stopped in the corridors by the head, who complained that he was wearing the wrong colour of shoes and also phoned his mother to complain about this.
They have a new head now, but if there uniform policy isn't changed, I won't be sending them there. A school which has time to badger pupils about what they wear, but has no interest in their academic abilities, is no good for me when I want my children to get good exam results.
I think half the problem is that uniform can help, IF coupled with measures to make students proud of their school and to make the school a better place for learning. But you can't just insist everyone wear uniform and expect it to make a difference when nothing else is being done to improve the school.