Гну.

Wearing a uniform, going to school, abiding by rules teaches children a healthy respect for authority and lets them know during those developmental years that they can't just 'do whatever the hell they please' and that there will be consequences to their actions.
If someone misbehaved in school and no one would own up to having done xyz... we ALL got punished! There are lessons to be learnt in every aspect of school life, your kids don't go to school to only learn their subject, they go to school to learn how to socialise, how to behave with other people and to turn out to be a well rounded individual... expressing themselves and wearing what they want, being who they are all comes later (or on weekends) but what sort of person will they be if they have no idea/respect/ or understanding of 'not being in charge'.

Plus, if they were allowed to wear anything to school you parents would be footing a much larger bill so that they can 'keep up' wiith the Jones'