Get a jazz teacher. The big thing the look at in these is style, sight reading, technique, and improvistation skills. Just bulk up on it all.

the best advice i can give you is practice, practice, practice. I have 2 jazz teachers on sax and 1 on trumpet, and they all say that, in order to improvise like a pro, you need to put your emotions into your solo. Think about the pieces you're playing, and what mood they give. Then take that feeling and play it. Also, get your scales and chords down cold. I'm in the same grade as you, so i'm pretty nervous also....i've always done auditons on trumpet, almost never on sax.....