A 2012 paper by Elisabetta Gentile and Scott Imberman (both University of Houston), published in the Journal of Urban Economics (ungated earlier version here), provides us with some evidence. Gentile and Imberman use data from "a large urban school district in the southwest United States" with "more than 200,000 students and close to 300 schools", and look at the impacts of school uniform policies on school attendance, the rate of disciplinary infractions, suspensions (in-school and out-of-school), and achievement in maths, reading, and language. They look at both elementary schools and middle/high schools. Importantly, over the period they look at (1993-2006):
Initially, only a handful of schools required uniforms. However, uniform adoption grew substantially over the following 13 years. Of schools that responded to our survey of uniform policies, which we describe in more detail below, only 10% required uniforms in 1993. By 2006, 82% of these schools required uniforms. In addition, no schools abandoned uniforms after adoption.So, we know there is sufficient variation in school uniform policies that we can essentially be looking at before-and-after comparisons within each school of the effects of adopting a school uniform policy. After controlling for student, school, and principal characteristics, they find:
For elementary students we find little evidence of uniforms having impacts on attendance or disciplinary infractions... On the other hand, for middle and high school students, we find significant improvements in attendance rates, particularly for females... female attendance increases by a statistically significant 0.3 percentage points after uniform adoption. This is equivalent to an additional 1/2 day of school per year in a 180 day school-year... For disciplinary infractions estimates for middle/high school students are similar to those for elementary students.In other words, there was some evidence that uniforms are associated with greater school attendance (for middle/high school students), but no association with discipline (including suspensions). Interestingly, they also find that:
...attendance improvements mainly accrue to students who are economically disadvantaged, particularly those who are in high poverty schools.Given that attendance improves, and improves most among disadvantaged students, does this translate into better student achievement? Unfortunately, their:
...results indicate that uniforms have little impact on achievement gains.There's also no evidence of an impact on students switching schools (either to avoid uniforms or to get into a school that has uniforms) and no association with grade retention. Overall, there is little evidence to support claims that school uniforms reduce bullying or violence. And while it might be good if school uniforms increase school attendance, that isn't much benefit if it isn't reflected in learning gains.