Discrimination based on having a womanly body? That's a new one. At any rate, the school has a right to impose restrictions as to how students dress. As you correctly discern, such rules should be applied, if at all, in a uniform manner.
Freedom of expression is a cherished right under the First Amendment to the Constitution. In my humble view, using the Constitution to get you the right to wear leggings just like the young women who have less mature figures is almost demeaning to the Constitution.
Let's first look at it pragmatically. Hundreds of hours of attorney time will have to be spent on your planned endeavor. I assume that you, as a teenager, don't have that kind of money to spend. And, somehow, I just don't see your parents telling you that the money they were going to spend on your college education is, instead, going to go to your lawyer so that you can wear leggings just like some of the other girls. I can also tell you that I seriously doubt there is any attorney in the world who wants to devote his or her time for free to such a silly litigation. And I don't think organizations such as the ACLU would be particularly interested in this issue.
Your second pragmatic consideration (assuming that you find a way to get past the first hurdle) is that by the time the case is adjudicated (if not dismissed early on), you will likely be graduating college.
Your third pragmatic consideration is that even if you are completely right as a legal matter, all you would have accomplished is to assure that your high school makes sure that nobody wears leggings.
Similarly, I don't see equal protection as a viable approach here, because discrimination based on female body type is less than likely to receive the court's "strict scrutiny," and, instead, merely a rationale basis level of scrutiny would be applied.
As for discrimination, body type does not figure in federal statutes. Some states have far more categories than the federal government does as to discrimination in places of public accommodation and the like, but I think it safe to say that no state will recognize a cause of action based on body type discrimination.
It's to your credit that you are thinking young woman. And that you are willing to fight for your rights. And that you have an inherent sense of fair play. But this is not the battle for you. You can easily bring your complaint of disparate treatment to the attention of the school administration, without engaging in First Amendment litigation.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as rendering legal advice involves the ability of the attorney to ask appropriate questions of the person seeking such advice and to thus gather appropriate information. In addition, an attorney/client relationship is formed only by specific agreement. The purpose of this answer is to provide the questioner with general information, not to outline specific legal rights and remedies.
Everything Mr. Haber told you is good advice. Also, even if this case did get litigated (and it never would), you'd lose. School officials get wide latitude from the courts to monitor and control students. Suspicion-less drug testing, locker searches without probable cause, etc. I'm sorry if all of this seems unfair, but at least now you know. Good luck.