I would suggest asking your band teacher or better yet your private teacher who has heard you play and knows your level of ability to select 2 or 3 pieces for you to choose from. It appears that you are a very advanced student for your age, congratulations on your musical accomplishments so far. Good luck with your festival preparation. Remember a music festival is a place to demonstrate what you can play, not what you can't. Don't pick a piece too hard for your level of ability.