Many public school districts have pointed to Catholic schools as evidence of the effectiveness of uniforms. Doing so ignores the basic point that children in private schools are more likely to come from families in which the parents place a higher emphasis on achievement, attendance and behavior. Curriculum differences at private schools can also be a bigger factor in academic performance, and private schools can more easily expel a student who does not conform to school rules.
Administrators in the Long Beach Unified School District in California have pointed to school uniforms as a big factor in improved performance and behavior of students. The problem with that conclusion, however, is that the district undertook other measures to improve student performance, but district officials were quick to attribute almost all the change to the clothing. Students themselves did not admit to any improvement in performance, nor did they say they felt safer.
One potential negative of school uniforms is that it allows some visible cues that would indicate potential problems to remain hidden. In the Long Beach case, students said gang members were able to conceal their affiliations. In other cases, a sudden change in a student's clothing can suggest a significant life change an educator would miss in a school with uniforms. Whether those negatives are statistically significant is not certain.
A University of Houston study suggested there was a small positive effect on language scores and attendance for girls in a large school district in the Southwest in which schools were allowed to require uniforms. In elementary school, there was little measurable difference anywhere. For boys there was a drop in reading scores for Hispanic students wearing uniforms and more disciplinary problems overall, though some of the issues may have been uniform infractions.