California: Stanford, UC Berkeley, UCLA, etc.
Florida: University of Florida, University of Miami, and Florida State
These schools (like Stanford) are some of the best in the country as well. With that being said, please realize you'll be competing with 3.3+ MILLION high school seniors for one spot. You definitely need to speak to your counselor and academic advisor to help you get everything prepared if you're serious about attending. It won't be easy, but don't give up
Also, start taking the SAT and ACT, there are some Americans who have been preparing since the age of 10-- no joke.
As for your other question, just to be sure I understand correctly: Are you asking if in order to take a certain class (Microbiology) in college, is that judged by your GPA/classes you took in junior/senior year (A-levels)?
If yes, here's the answer:
When you go to university/college in America, regardless of what you studied during A-levels/high school, you can study ANYTHING you desire. Medicine, Engineering, English-- ANYTHING. Major, double major, minor in it, it does not matter. Your money, your choice.
HOWEVER, certain majors require you to be accepted 'twice'. This is what I mean: My brother wanted to study engineering at a certain school. First he had to be accepted into the university, then be accepted into their engineering school. Using your SAT/ACT scores, GPA, extracurriculars, interview, recommendations, and a few other elements, will decide if you're accepted into these specific programs. Not all courses require prior acceptance, but I know Engineering and Medicine probably do.