Can I attend Harvard Medical School if I attend Harvard College or do I have to go to another school?
Harvard Medical School has their own applications process and it’s completely separate from Harvard College. Just because a student is admitted to Harvard College does not mean they are automatically admitted to HMS. All Harvard College graduates must apply to Harvard Medical School just like everyone else – and many (most) are rejected.
Have you been admitted, are you attending, or are you a high schooler?
@compmom: In another thread, @smusername495 identifies self as first-year in high school.
Then smusername, you are waaayyy ahead of yourself 
Enjoy high school!
Although they’re a bit premature, I believe the OP is asking a legitimate question, that is, does Harvard’s medical school discriminate against students who earn their undergraduate degrees at Harvard?
I believe the answer is “no”, but I do know that in some fields, specifically Physics, many schools prefer their undergraduates go elsewhere to pursue their PhD studies.
You believe correctly, and this would apply to other professional schools (but not GSAS) e.g. HBS, HLS, etc.
Same is true for Harvard for most PhD areas.
But the OP is a freshman in HS, and as others have mentioned, is waaaay ahead of him/herself. Focus on undergraduate admissions (although really, don’t put too much energy into it until junior year, other than maximing your studies and ECs).