What (if any) books would you ban from a high school library? Are there certain subjects that you feel are inappropriate for teenagers regardless of literary merit?
None. There are of course limits to what such a library can hold -- both budgetary and space-wise -- so there will of course be some element of "banning" in a de-facto sense because the librarians will have to choose what books they BUY for the library.
However, if we assume that I have an infinite library so all published books fit in there, I ban none of them. I may separate some out into an area where they are clearly marked as "your parents may not want you to read these", but maybe not; after all, that would just invite them to borrow the books MORE.
A librarian's job is to provide access to information, not to decide what information the library provides. There may be SPECIFIC CASES -- as in, relative not to the book's content, but to the individual accessing said book -- in which you may want to either not hand the book over, or at least notify responsible people of the possibility of a problem -- but the book itself should not be, in effect, penalized for the fact that in the wrong hands it can cause trouble.
INFORMATIONAL EDIT: I will note that back in... 1992, I was in an "Ethics in Information Science" class, and this precise question was asked. To my astonishment, I was either the only one, or one of only two, people in a class of about 30 would-be librarians and information professionals, who answered "no censorship" in the absolute and complete sense. All others were willing to do SOME level of censorship, ranging from "I wouldn't buy such books for the library" to "I'd file them so they're hard for anyone to locate" to "I'd have those books removed from the library as soon as possible".