Science magnet is a high school ? (I have no idea.) How are other students "super prepared" and you're not? Have you taken any university science? My point is that you could be underestimating yourself.
I agree with the previous poster that thinking about a postbac at this point doesn't make any sense. If you want to be a physician then get going. If you have yourself convinced that you can't get A's in science at your current school, then think about changing schools. You could also think about finishing your current degree, and then going into the Peace Corps or similar. If you still want to be a physician when you get home, do a postbac then.
Either way get your fanny in a hospital and start volunteering. Right away.
Also, this is pre-osteo, and we don't wet our pants over getting straight A's here. If this is a miraculous "DO is better and I'm not interested in MD" case, then hallelujah, but I suspect you're thinking DO's easy to get into. Not so much.
Best of luck to you.