I would reccommend not attempting to perform the Bartok concerto until at least college. It is one of the hardest and most complex pieces in the repertoire. You should discuss this issue with your private teacher if you have one, or your orchestra director. A really good young student piece is the Stamitz concerto in D Major. A lot of people don't like it because they think it is dull, but it requires a lot of skill to play well and it also has some very sweet melodic sections which show off good tone. The J C Bach concerto in C minor is a really good piece as well. Also, try the Telemann concerto in G major. Order your music from a local music store or online.