I have looked at this from many angles, as a girl at grammar school, having to wear skirts and their very expensive blazer in the summer, That was a great expense and hardly got worn, the skirt rule meaning that you froze in the winter time, and cold legs did not help you to concentrate on your work. As a parent of a child that grew very quickly and having a set uniform , meant a lot of expense, as nothing was worn out but grown out of very quickly and these set uniforms never had the kinds of hems that you would put on yourself.
As a teacher , being expected to police the uniform rule, when quite often I had every sympathy with the children having to wear something that would be too warm or too cold, and was something that was just a distraction from learning. Clothes also might be meant to make children look the same, but it would be clear to see who had new clothes and whose were hand me downs etc.
My youngest sisters school had the best idea, which both worked and was simple. The children had to wear white shirts, no tie, and grey trousers, or skirts, and jumpers. So all the school were wearing the correct colour and then there was a badge that could be sown onto the jumper or whatever. That way meant that a child could wear whatever was comfortable and suitable for that days lessons. The colour meant that the clothes were easily available at large stores, orcould be hand made etc. It worked well, cost less, and of course, did not become a way to show rebellion , just your own choice to be comfortable doing your work.
If school governers were made to wear skirts with ankle sox and shoes in the middle of a cold winter for a week, they would quickly come to their senses and at the very least these days allow tights or leggings. To have children concentrating on their lesson, rather than thinking about being too hot or too cold , is more important than clothes.