According to the American Library Association's (ALA) web page (2016):
"Throughout history, more and different kinds of people and groups of all persuasions than you might first suppose, who, for all sorts of reasons, have attempted - and continue to attempt - to suppress anything that conflicts with or anyone who disagrees with their own beliefs.
"In his book Free Speech for Me - But Not for Thee: How the American Left and Right Relentlessly Censor Each Other, Nat Hentoff writes that 'the lust to suppress can come from any direction.' He quotes Phil Kerby, a former editor of the Los Angeles Times, as saying, 'Censorship is the strongest drive in human nature; sex is a weak second.' "
"According to the Challenges by Initiator, Institution, Type, and Year, parents challenge materials more often than any other group."
The links below are mostly for the educator's own knowledge, but the sources could also be used as a spring board for discussions and assignments for middle school on up.