What can I partake in hs that will help in dental school?

Hold on there. It's good that you are
already primed about getting into the profession, but you are overlooking the
college aspect of things.
Getting into a Dental school will depend
on your COLLEGE gpa and DAT scores. But getting into a good college depends on your
high school gpa amongst other things ( so definitely get rid of the C's )
but regarding extracurriculars, volunteering in general and 'shadowing' a local practitioner will be quite an eye-opening experience as far as getting a better idea about what goes on in a dental office.
also building a relationship with a dentist can get you a job as an assistant or at the very least a nice recommendation when the time comes. Don't overlook the dental labs in your area as well, as the technicians there are doing what you'll likely be doing in your first 2 years of school.

When you get to the dental school application . . . extra curriculars will count as far as your ability to be a team player, a leader and a community oriented person.( those are great qualities to have ) In addition, any activity which improves your manual dexterity ( especially playing a musical instrument, artwork ( painting etc ) ) and communication skills.

Since you have such an early start i don't see any reason why you can get a comprehensive view on what dentistry encompasses.

Good Luck.