well, lots of good general advice here, though i'm kinda down with windhorse in the 'what matters is now' ethos.
a couple of thoughts...
1) get a tutor. just because you're very bright doesn't mean you can test well. i never did well on tests. well, actually, i did, but only in subjects i liked. a great tutor can help you compartmentalize your thinking and master the task of test-taking.
2) figure out what kind of learner you are. some people are auditory learners [learn by listening], some are visual [seeing], some are kinesthetic or tactile [doing/feeling/experiencing... being 'hands-on]. you may not be able to customize every subject to your learning style, but recognizing your strengths can help you zero in on your weaknesses and set up a learning system that helps you work with what you've got.
3) make sure you don't need glasses or a hearing aid or something like that. it's hard to recognize those things when you're younger, and they really have a huge impact on your ability to learn.
4) Try to learn conceptually rather than by rote. If you can articulate the subject matter in a few paragraphs rather than try to retain everything on the page, you'll get a better "big picture" sense of what you're learning, rather than meaningless bits of data that don't add up to anything to you.
5) Apply your drumming to your life and schooling. if you are truly a disciplined drummer, you can carry that same attitude over to your studies.
5) When you're ready to graduate, apply to Goddard College. They don't do grades there.

i have known a LOT of extremely successful people who never went to college, or did well in school. there are other ways to contribute to the planet than just being a good student. even einstein failed math. BUT -- he did say that 'genius was 1% inspiration and 99% perspiration,' so even mr. e=mc2 recognized that talent without application doesn't net you much.
so, no -- grades aren't the end-all, but at the very least, good grades will open up your options, and give you a sense of accomplishment when you see them go up. and if you are naturally smart, you'll want to learn just to learn.
smoggy
[who never was able to get anything above a C- in math, and it really ticks her off!!!]