Useful reading can be had in the USSC's decision in the Pico case. The decision was 5-4 against banning books in school libraries.
Here are the majority and minority opinions:
http://www.firstamendmentschools.org/resources/handout1a.aspx?id=13965
... In brief, we hold that local school boards may not remove books from school library shelves simply because they dislike the ideas contained in those books and seek by their removal to 'prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion.
Both groups, however, seem to agree that the school shouldn't remove books from the library for ideological reasons, if there isn't say, a public library in the neighborhood that carries said book.
So anyway, following the Pico case, school districts may not ban books from school libraries, but it appears to happen all the time, nevertheless (perhaps until challenged in court?).