If I may weigh in as a relative newbie to this board....
Yoshi, you're clearly a junior or a senior in hs, from what you say. If you are looking for parts in the future, you may want to think twice before you quit the show. At my kids' high school, kids have to sign a pre-audition form stating whether they'd be willing to accept any role. If they say yes, and then quit the show, they will never be cast in any other show for the duration of their time in school. This forces kids to be honest with themselves and with the drama/music teachers, and minimizes diva behavior.
My son just went through this -- he said he'd take any role, then didn't get the role he wanted in the fall musical. He was very upset, thought about quitting. Then he sucked up his attitude, took his spot in the chorus, had a wonderful time, was supportive of the guy who got the role he had wanted, and moved on to the spring play, where he got exactly the role he wanted.
I would also encourage you to speak to the director very honestly and openly and ask why you haven't gotten the parts you wanted. That's a tough and brave thing to do, and you will probably learn a lot, if you listen. And it's an opportunity you will *only* get in school. Once you're out in the professional world, if you choose to go in that direction, you will never get the chance to learn why you didn't get a role, so take advantage of the opportunity now while you're in a student-teacher relationship!