Everyone makes mistakes. Be ready to play very softly and very loudly or soft and loud. If you don't get the chair you want, don't sweat it. Ask your teacher what skills you don't have and work those. Ask if you can retest in a month or two.
Some schools have music coaches for free.
Get one if you can. If you're first or second chair, try to help your section members improve. Have practice and pizza parties to make working together fun. Get into concert band and jazz and as many groups as you have time for. Try different styles.