If you don't want to be perceived as a sports type person, then best thing is to dress and act as to how you want to be treated. If you bring a laptop around or pocket pc, then maybe people will get the hint. Of course if you already have all the next test question figured out, that means people will figure out that you must be a nerd.
I had a cousin that was over 6 ft tall when he was 13, people thought he was made for football, until most people learned to run around him (being big is also being a bit slower sometimes).