School uniforms help avoid distraction.
Non-uniform regular clothing can be a distraction for students from the activities of learning.
When uniforms are mandatory, children behave better in class. This is likely due to the creation of a sense of hierarchy which reinforces discipline.
When students are forced to wear different clothes to school, the uniforms serve as a constant reminder that the students are not at home and there are behavioural expectations they are required to meet. This leads to better classroom behaviour. Part of this is due to the clear establishment of hierarchy. Students are required to wear a school uniform, while teachers wear different clothes. This visual demarcation of authority serves to internalize a hierarchy and instil discipline. 
By this logic, schools in the United Kingdom, where almost every school has a mandatory uniform policy, should have markedly improved classroom discipline than schools in the United States, where the majority of public school students are not forced to wear a uniform. But this is not the case. In a study published in the Journal of Cross-Cultural Psychology, the US and the UK showed similar levels of classroom discipline, indicating that the existence of a mandatory school uniform policy had no effect on discipline and did little to reinforce a classroom hierarchy. 
[P1] School uniforms help reinforce behavioural expectations and establish hierarchy. [P2] This leads to an improvement in student behaviour. [P3] Therefore, school uniforms should be mandatory.
[Rejecting P2] Stats do not indicate any improvement in behaviour.