Students should wear schoo uniforms to avoie distraction and conflict in school. The uniform provides an equalizer across all students. There is no fighting or talking about people based on the name brands of clothes being worn by the students.
The purpose of having school uniforms is that it standardizes the dress code. This of course is helpful for parents not fighting with their offspring about what is appropriate school clothing. Another reason is that it takes away the divide between the haves and the have nots.