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.