Well, on the most superficial level, I don't think you should do it because you are still wearing a piece of someone else's culture as a costume and that is against your school's rules. You're going to get in trouble anyway.
That said, for me to decide whether or not the idea of you wearing a kimono would be offensive, I would have to know a few things: Do you actually know HOW to wear a kimono? Do you know the rules for wearing one, such as how to match the kimono to the current season and how to choose appropriate accessories based on the occasion AND the formality of the kimono? What kind of kimono do you plan on wearing -- do you have a real one or just a kimono-shaped bathrobe? There are lots of rules to wearing kimono, do you know ANY of them?
You also need to learn about cultural appropriation. I'm assuming you're not even in college yet so it might be a tough concept to grasp. Plenty of college-educated adults struggle with it. But I would suggest that you start by reading through the Wikipedia article on cultural appropriation to get an idea of what I'm talking about. But at the end of the day, a white person deciding to turn the cultural clothing of a minority group into a costume is in NO WAY equivalent to a member of a minority group wearing a ballgown. Life isn't fair, as many of these people can tell you -- white people have a long and sordid history of stealing minority's lands, destroying their languages and cultures, tearing children from their homes, and trivializing the things that minority peoples have done to try to maintain their cultures. One way we as a group do that is by taking things from other groups and using them WAY out of context, for example, pretty much any instance of a white person (especially a woman) wearing a Plains Indian war bonnet. Unfortunately the thing that makes cultural appropriate so tough is that the line between borrowing and using things APPRECIATIVELY and simply APPROPRIATING something and using it inappropriately is very fine, and people disagree on where that line actually is.
Your right to express yourself is NOT being denied. You are just being told that you can't express yourself in a way that offends other people. This is NO different than being told you can't wear a t-shirt with the F word or the N word on it to school. Also, consider this: How would the people around you react to a Japanese or Japanese-American student who wore a kimono to school on some day other than Halloween or some other day specially set aside for ethnic clothing? If the answer is that nobody would even notice one way or the other and that student would be allowed to go about his or her day without being treated differently for it, good for you, you happen to go to a very enlightened school. But chances are that person would get a lot of attention, not all of it positive. So why should you, who by your own admissions is not Japanese, get to wear kimono to school any time when they probably couldn't without risking everything from stares to violent bullying? THAT is what is unfair here, not that you are being told that certain costume choices are off-limits because they might offend someone.