I am a former High School Track Coach and Pac-10 level Track and Field and Cross-Country runner. Your time is excellent for a 15 year old. Running the 10K takes a lot of strength and generally 15 year olds don't have it yet. I would concentrate on the 5k and lower and run on your high school cross-country and Track teams. Don't worry about the 10k until you are much older.
Well thats the same as running two 18:46 5k's in a row which could probably get you on a decent varsity team let alone the fact that you could probably run mid 17:00's if not low 17:00's or possibly high 16:00's depending on your footspeed. For a 15 year old you're looking towards a bright running future.
Yes that is a good time. your average mile is a little over 6 minutes which is what I run a 5k in, and i was about average freshman year. keep it up!
Sounds pretty average to me honestly.