you know, that's a tough question, actually. If you've got three categories to choose from, I think people will disagree about where lots of schools get placed. Probably no one would disagree with putting Harvard in the upper tier and the Carribbean schools in the lower tier (I mean, I don't know, maybe someone would disagree with that), but a lot of the others, who knows? Different people will have different criteria for where a school gets placed, and not everyone would rely on a ranking system like U.S. News.
An example: I saw a Wall St. Journal article recently that was discussing "prestigious" medical schools, and among the five or six it mentioned was Dartmouth. Now Dartmouth comes in at #34 on the U.S. News list, which a lot of people would probably call mid-tier, yet whoever wrote this article considered it a top school. So the interpretation of the tiers will probably vary based on who's talking and what they're considering important.