Case Study 1- Long Beach California: comprising nearly 60,000 students, this was the first large urban school district in the United States to require school uniforms for all students.
Since 1994,
when mandatory uniform policies were adopted in this school district, officials have found that violence and discipline problems dramatically decreased. In the first year following implementation, overall school crime decreased by 36%; sex offenses, by 74%; physical fights between students, by 51%; weapons offenses, by 50%; assault and battery offenses, by 34%; school suspensions, by 32%;
and vandalism, by 18%.11

http://instruct1.cit.cornell.edu/courses/pam230/Paper%20Examples/PAM%20230%20Example%20Paper%201.pdf

Enter the poor man's friend, The ACLU:

In October 1995, working on behalf of low-income families, the ACLU of Southern California filed a lawsuit against the Long Beach Unified School District. The lawsuit claimed that the district fails to help low-income students purchase uniforms and has punished students who do not wear them. It also claimed the district does not adequately inform parents about their rights to request exemption from the program. ACLU attorneys assert that low socioeconomic families are going without food, utilities, and rental payments in order to purchase mandatory school uniforms. In response to these claims, Long Beach Unified School officials state that the district has spent more than $100 thousand in donations from individuals and organizations to purchase uniforms and other supplies for financially burdened students. The officials quickly point out that typically, a set of three school uniforms for the year costs between $ 70 and $ 90, an amount far less than many students spend for one item of designer clothing.

http://www.danenet.org/ncs/forumuniformseval.htm