I know that my high school choirs did District Music Festival once a year. I think all the choirs did it, but students also had the option of doing a small ensemble (3-8 people, mostly self-directed), or solos.
Now, most people in my high school who opted to take solos WERE in private voice lessons (and if you've got the time, money, and can find a decent voice teacher in your hometown, I'd highly recommend it... there's nothing like one-on-one tutoring with a professional), but those who weren't just spoke with whichever choir director they were closer to, and the director picked out some songs for them. Then, they were pretty much on their own. Oh, the director listened to them once or twice and gave constructive criticism, but that isn't really a substitute for a once-a-week session with a private tutor. ._.;
But... yeah. Ask your director. S/he should know what to do to get you in.