This piece from Education Week discusses national research studies on school uniforms.
David L. Brunsma, sociology professor at the University of Missouri-Columbia conducted major national studies. Bottom Line: "...uniform policies don't curb violence or behavioral problems in schools. They don't cultivate student self-esteem and motivation. They don't balance the social-status differences that often separate students. And they don't improve academic achievement. (In fact, uniforms may even be associated with a small detrimental effect on achievement in reading, his research shows.)"
Journalist's Resource offers a collection of abstracts from several studies. They include a slew of reasons to doubt the value of school uniforms:
"The results do not suggest any significant association between school uniform policies and achievement."
"Contrary to current discourse, the authors found a negative effect of uniforms on student academic achievement." - (this is also from Brunsma)
"Although the effect size was small, students from schools without uniforms reported higher self-perception scores than students from schools with uniform policies."
Also included is a reference to an article criticizing some of Brunsma's methods, and a small scale study which reports some positive effects.
I dug up the full paper on that that study. It examined one single school district, and most of the reported benefits were attibuted to one segment of students. The researchers themselves outline a number of limitations in the data, some of them significant - such as not having centralized records of the time when schools began requiring uniforms.
Also significant the last study mentioned in Journalist's Resource, which looks at the role of uniforms on spending. Uniforms don't replace other apparel purchasing or spending. "Rather, uniforms and nonuniform apparel appear to be complements in consumers' purchases, resulting in greater household expenditures on nonuniform apparel."