Some schools require their students to wear uniform but some allow them to use casual wears. A school uniform has many advantages, but also has some drawbacks as well.
School uniforms help students and teachers to forget about difference in status among individuals. Although students come from different families and different socioeconomic levels, when they are at school, they are all ordinary students. Kids don't have to worry about clothes and be able to concentrate on learning.
Uniforms can also help teachers or caretakers to take care of their children. Teachers are able to know at a glance that the children are within their responsibility.
There are some disadvantages of uniforms as well. Obviously, it limits the freedom of the students. For example, some girls won't be happy to wear skirt uniforms and feel more convenient with pants. Different persons have their own styles and there is no "one size fits all".
School uniforms are also limited in use compare to casual wears. We rarely wear uniforms for purposes other than for schools. But casual wears can be used in many situations.
In my opinion, it is up to different school policies and culture. Uniforms have both advantages and disadvantages.