Funnily enough, one of the reasons they told us to wear a uniform was for school excursions. If the class or year were going on some sort of excursion, the school was being represented, and wearing different clothes by the students was meant to be a slur on the school.
For about the first 2 years I was in High school, I wore the uniform (except for the shoes) and it was toward the end I added my own variations. 3rd form and later, I couldn't be bothered with it, and wore wtf I liked. Got sent to the principal a few times, and got sent home once to change, but always used the excuse I didn't have any uniform pants or jumper etc. They gave up after awhile. Also, the school claimed it was cheaper to wear uniform, but when you wore normal clothes every day, that sort of blew that argument out of the water.
Primary chool were never big on uniforms anyway.