Reading around this subject there appears to have been a 1998 summary in the US which is quoted a lot in political debates full report here.. This research summarised that
Our findings indicate that student uniforms have no direct effect on
substance use, behavioral problems or attendance. A negative effect of
uniforms on student academic achievement was found
Which seems to be opposite to the prevailing wisdom about uniform promoting good behaviour.
There is also a study from 1998 study Into things which effect behaviour ins schools with a specific section on schools that can be found here.
This study also concluded
The notions that school uniforms and zero tolerance for gangs would
reduce school disorder and consequently improve student academic
performance were not supported
Looking for contraindications to the above evidence there is an article quoted around a bit from psychology today which indicated that uniforms had a huge positive impact.
However I suspect there is a correlation not the direct causality indicated there (but without paying I can't access the full article). Indeed one of the quotes from people associated with the schools said this
Dick Van Der Laan of the Long Beach Unified School District explained,
"We can't attribute the improvement exclusively to school uniforms,
but we think it's more than coincidental."
I think this quote highlights one of the problems with researching the issue of school uniforms. School's do not exist in a vacuum, and a whole host of things change from term to term when there is a problem with behaviour in an attempt to sort out issues.
It is difficult to tell whether benefits are anything to do with the school uniform itself, or other reforms which happened around the same time. One can also guess that schools with strict dress policies and/or uniform policies will also be more strict on behavioural terms (just being consistent) so any study just considering uniforms may be skewed.
Anecdotal evidence is available on the department of education website too. But there still seems to be a lack of direct evidence. It can be noted that this page indicates that schools should
Treat school uniforms as part of an overall safety program
And not treat it as a single fix for anything.
Actual statistical comparisons seem to be thin on the ground apart from the first two studies, and most of the stuff I read calls for more studies, however I can see difficulties with producing comprehensive and verifiable information about the effect of uniforms in general.