Are you referring to student uniforms? If so, generally it is the dress code of the attending school. Many schools feel as though a student should be focused on their studies instead of concerning themselves with the distraction of one's clothing.
It is efficient for the parents. Much time is spared by purchasing uniforms opposed to searching for miscellaneous clothing items. It is also less expensive than purchasing casual wear.
There is also the benefit that the students are neat, tidy and represent the school in a respectful manner all dressed accordingly, it removes controversy of some choice clothing.
Although, many feel that it is denying the student the freedom of expression, it actually does create a better learning environment for the student and they still can choose and express themselves outside the school hours on their free time.
There are many valid reasons behind keeping dress code for kids. The main reason is that it promotes equality otherwise the competition of wearing best cloth can create a feeling of hate and jealousy. Another main reason is professional appearance. Children in a proper dress code take school more seriously. A uniform also promotes sense of unity, build the school spirit and enhance school pride.