I ran my first marathon in high school at 17. I did it in the 3 weeks between the cross country and track season. My CC coach was a joke, so I marathon trained during the CC season. He wasn't happy about it, but it actually seemed to help my CC races. However, indoor track started 3 weeks after my race. Looking back, I should have taken winter off. Oh well! I ran my first half-marathon at 14. I really think that because bones and muscles are developing so much that it is best to wait until you're almost done growing to do a full marathon - though many say to wait even for a half. At your age - maybe concentrate on just halfs for now. That way, you're less likely to get injured and can participate in high school sports.
*I didn't finish growing until 22 - REALLY late for a girl! However, I'm glad I ran my first marathon in high school, as I didn't know if I'd have the time later. (I ran my next one at 18 however - I should have taken more down time - that one wasn't a good idea).