MY advice at 15 . . . . is unfortunately to not get your hopes up.
You may be good fro your age, but at 15, there's simply going to be people older, more experienced, and ebtter than you.
I know it's disappointing to hear, and im really not saying dont audition, if you DO audition, you'll know whats coming for 08
You could always come to Memphis Sound with me =D (I'm 15 as well)
You reallu just need to make sure you go in with a great attitude.
Ready to learn, and read to work are tow key tihngs any corps will want.
Make sure that you are able to learn quickly.
Chances are your director has not cared about the style of roll/glide step you use. Spirit will.
Go in with the right attitude, work hard, and who knows, you jsut might make it!
Good luck.
Also, read Feet Guru's thread, it really helps.