There are some important points you address when considering why schools have uniforms.
As your side, the side of no uniforms, you have to use philosphically positive points (points which have unsupported data) for positive reinforcement.
However your opponet has far too much information on their side, most of it created and researched by "the man".
let us consider first how to win this arguement before we worry about being right...being right only matters when you lose, and even then in a much smaller degree then winning.
Your audience is your piers I'm assuming? Or a panel of judges of some kind. Your best strategy is first to consider your audience.
Peers (other students en mass not being graded by their response to your arguement):
Present your positive arguement about freedom of expression. You will need to answer their questions regarding statistics of increased safety from uniforms and everyone being on the same playing field, aka no kid left out. You must answer those questions with an almost joking tone...regard the concept of "fitting in" and "everyone on the same playing field" as fundemntal flaws in a free enterprise economy, and that this concept is more about creating a false sense of economic and social equality vs. preparing students for the real world after school.
Oh yes...and don't forget to throw in a few quotes about what happend when we lose our freedom to choose...I'd go with the one about "First they came for the ______ and I did not say anything. Then the came for _______, and I remained silent. After that was the ______, and I wanted to speak, but then they came for me...and there was no one left to protest." equate that to the uniforms, then add in some other freedoms that could eventually be taken away if we keep going down this "dangerous path of restriction of our right to the freedom of choice and expression."
Their only response should be to call out about obscene displays, and to that remind them that in all cases obscenity is an illegal form of harrassment, but preventing the wearing of clothing will never stop the determined to create havoc and sew mischeif.
And if they go the route of "well we can identify kids better this way and see who doesnt belong", point out that there is a responsibilty of the students to report strangers, and the faculty to know every kid in the school and to report strangers as well...stopping people to ask questions is better then putting them in a uniform easily duplicated
Teacher & small panel of judges being graded on their response:
Questions! Questions questions questions questions! You want to myre the concept of change and uniforms with SOOOOOOOOO many unanswered, and if possible, costly to answer questions.
For example...statistics exsist for increased security with school uniforms, what is the control group they are using?
Did they take into account the crime in the surronding area?
The average history of violence of every child in that school? The "environmental" impact at home that created this behavior from the violent ones in the first place?
Were the statistics created or funded by those who were selling items to schools to help increase security and control?
Here is a rigged question...after asking them about all these questions throw in one like this:
How do you measure the loss of an inalienable right that the constitution protects?
Rig the questions so that the only answer is to ask another question, at which point your rubtle is to point out that the question you have are unanswered, and there fore further research is needed before a change is made, or we could make the change and throw the negative concepts to the wind.
If they beleive there are no negative concepts...have a ping pong ball with you. Toss it to your opponet. If they catch it point out "Every action has a reaction, I throw this ball, you caught it." Ask for the ball back, and then toss it down the aisle. Watch it roll long enough for dramatic effect. Point out that this ball represents the negative side of uniforms and that currently "No one is catching" the ball, because no one had it tossed to them...for obvious effect point out that the research does not mention the negative sides, such as student morale/happiness...but most importantly, that the studies cannot measure creativity and expression as those are arts to which no restrictions should ever be made.
Visual gimicks create moments to which visual learners will cling to. Now you are engaging auditory, and visual, learners with your action...the more memorable it is the more they will remember the arguement attached to the visual action. By the way...don't overuse visual gimmicks or you turn into a clown.
I'm sure you can think of plenty more...but for every questions you think of, try and answer it as well. If the answer is logically and makes sense...pick a different question to ask. You want to present questions to which the best answer is "Perhaps we should research this further." That makes the inference that with further research we could study both side of this coin, and while you may not prove your point, you show theirs as rushed and hurried and without consideration to your sides concerns.
The words "Further study needs to be done" has delayed many projects and plans through the history of democracy, some to the point that they just fade away.
On a side note: debates are never won in the opening statement, they are always won on the rebutle. It is the points of your opponet that you refute that grant your arguement more weight. If your school does not have uniforms then your already on the winning side and it is up to your opponet to defeat you. Ignore the minuta they want you to engage on and focus only on your core points and their core weaknesses...if they throw a good curveball, don't swing and say the following "You have brought up a good point, however further research needs to be conducted into the negative effects of school uniforms." Or something like that...you get the point.
If your school already has uniforms you need to appeal to the rebel and show that without them very little would change. Show that there are other ways to keep the student population safe without uniforms, that a fear driven decision should never restrict freedom (your teacher will love that statement), and that while parts of the real world require "proper attire" the goal of school is to prepare people for the real world...all of it...not just the part that wears uniforms.
I hope that helps...I apologize if its repeatative.