I think that just about any school is going to give you exposure to a "broad enough" range of diseases -- they have to because their accreditation hinges on having clinical affiliates with a broad enough range. Every school tries to have a few special patients for teaching purposes. (I can recall in the 1980s when the US Public Health Service wanted to centralize treatment of Hansen's disease patients in NYC and every derm department dug in their heels because they liked having one or two that followed up with them annually "for teaching purposes".) Now, as a student, you might not be on that clerkship or elective at the right time of year to see the rare patient with X or Y but in all likelikhood you will see a wide variety of the "bread and butter" cases in that specialty. In residency and fellowship you'll see a broader array within a more narrow field.