Non-uniform regular clothing can be a distraction for students from the activities of learning.
Students are at school to learn. Anything that serves to distract students should be removed from the classroom environment to foster and aid retention.
If everyone is wearing the same outfit then there is less of a distraction. In places where school uniforms are mandatory, students are more focused on their work and less on their attire. In one study carried out among elementary school students in Houston, elementary school girls language tests increased by 3% after the school introduced mandatory school uniforms.
This isn't supported by evidence. A comprehensive study carried out by Virginia Tech's David L. Brunsma found that uniforms had no effect on academic performance or student behaviour in a national sample of 10th-grade students. Quite the opposite. When all other factors were controlled, Brunsma found that a school uniform had a negative effect on academic performance. When he repeated the test on students at elementary school age, he found similar results, indicating that school uniforms do not improve academic performance by removing distractions.
[P1] Wearing street clothes is a distraction. [P2] Schools should aim to eliminate distractions. [P3] School uniforms should be mandatory.
[P1] Wearing street clothes is no more of a distraction than wearing uniforms.