I don't think many schools have a "premed" major and even if they did, there's no good reason to do that major. Med school requirements can be met through almost any major - typically one year of bio, orgo, chem, phys with lab as well as english and a few upper level science courses. Major in something you're interested in, and if it's biology or chemistry, so be it. More and more applicants are applying to medical schools with humanities majors - they definitely get more "attention" or at least they have more to talk about at interviews, but again, don't choose a major based on what you think will increase your chances of acceptance. Just excel at whatever you choose. As for science course grades, it all depends what kind of medical school you want to get into, but you definitely want A's and B's for all science courses. If not, it puts you at a severe disadvantage, requiring you to really shine in other areas of your application. Many see people college simply as a means to an end (med school). Not surprisingly, they're the ones who enjoy their college years the least.
I wouldn't say that people classify schools as being good "premed" schools. Medical school admissions committees do take into account what school you went to, but that is based on the school's overall reputation, not just the premedical aspect. As for difficulty, it's all relative and whether or not it's hard to get into a good "premed" school depends on what kind of person you are and how you've done in high school. Getting into medical school is much, much harder than getting into college. Getting into a good residency is even harder than getting into medical school (varies with specialty). Medical school is four years. Residencies range anywhere from 3 years (Internal Medicine) to Neurosurgery (7 years). This is not counting any fellowship work after residency for board certification. So including undergrad, you're looking at 11-15+ years of post-HS work. Jasmine's comment about "one year or so of residency" is referring to the intern year, after which you can be licensed to practice (only as a general practicioner). Going this route is not very common nowadays and you will have difficulty getting a good position and cannot be board certified. Almost everyone goes through an entire residency training program.
It's great that you're planning so far ahead, although there's really no need to sweat these kinds of details at this point in time. Doing well at any reasonably recognized college and keeping the same intensity throughout while maintaining a "big picture" perspective will serve you well.
Good luck.
