Help With Changing School System Rules?

Is this the right place for this? Idk. Anyway, I am in highschool and I've been out of the closet since 8th grade. In 8th grade, all of the students go on a trip to Washington D.C. but I wasn't allowed to go because I was gay. At that time, I didn't want to do anything because it was all really embarrassing for me. But now a few years later, my friend's brother is having the same problem, and I really think it needs to change. But my question is, uh, where do I start? And what would be the most effective way of getting it noticed or changed? My school has a small student led g.s.a. group and I'm planning on telling them, so I know I have a group of at least 20 supporters. Thanks.