I think most premeds knew early on that they wanted to be doctors, but I think high schoolers need to take the advise of current pre meds/medical students to heart. I agree with most posts stating that anyone in high school should just try to enjoy as much as possible and not focus on what they can do now that would improve their future applications.
I am a current med student and I knew when I was in high school that I wanted to be a doctor, but did I take anatomy, physics, etc thinking that it would somehow help me in the application process....NOOOOOO!!!!! The stress that you will have in college and all the premed classes that you would have to take is enough, and I would strongly suggest that you enjoy art, music or whatever while you can.
If you have a true interest then take AP biology, anatomy or whatever, but honestly try to enjoy your life as much as possible. It's great that some of you are shadowing and getting clinical exposure, but know that it really wont do anything to improve your application. The coming four years in college will determine your aspirations more than what you do in high school. You will also find that the majority of premeds in college, that were dead set on becoming doctors, will chose something else.
Good luck and have fun!