Not actual quantitative data - no. But anecdotal evidence that it doesn't make much of a difference. When I arrived here (at my school) there was no uniform beyond them being asked not to show too much flesh or advertise alcohol (which also meant no to many football strips). A uniform, of sorts, was introduced: black trousers, skirt or shorts, black hoodie (school bought or your own plain black one) and a white t shirt or shirt.
Nothing has changed. The pupils are the same - the ones who pushed the boundaries before (t shirts with rude slogans - my personal favourite "I don't drink water. Fish fuck in it!") are similar to those who now wear a navy hoodie or whose white t shirt is permanently in the wash.
Results are the same. Behaviour is the same. Pupils are the same.
The only slight issue I have with it (and the rules are very, very lightly applied, if at all, by management) is that it creates bad feeling. As a parent, I am pissed off by the poor quality hoodies. My children don't see how black jeans make them better than ordinary ones. As a teacher, I spend more time nagging and cajoling than I did.
I came from a background of very, very strict uniform (school issued and free so everyone had the same and you handed it back and got more when you outgrew it). I always felt strongly in favour of uniform but now, not really.
I actually miss the pupils showing off what is in fashion. I miss their chance to be individuals without breaking rules (they now only have to wear a scarf, red t shirt etc to break the rules).
Childhood / teens are horribly short and I quite like the idea of them being given some freedom. My children wear it (they have to - I teach here) and I nag pupils but I do both out of a sense of duty rather than because I feel it does anything at all to improve results.
We have had pupils go on into all the professions. They cope, just as soldiers do, with a new "unform", just as they cope with moving into retail and wearing a uniform.
Would abolish it tomorrow were I Queen!