An example of a controversial book and its challenges.
Parents of a sixth grade student at Sage Elementary School complained that the book The View From the Cherry Tree "contain[ed] vulgar words...and with the way this book would teach 12-year-olds that its OK to treat elderly people like the lady in the book was treated." One of the specific vulgarities that the parents took issue with was the protagonist's pet cat's name: S.O.B.
This book was recommended for grades 5-9 at the time is was published. In reviews by The Booklist and Bulletin for the Center for Children's Books it was lauded and described as "solidly constructed" and with excellent characterization.
When I researched this book I found it listed as the 97th most challenged book of the last decade. When it is challenged it is due to violence (there's a murder in the book) and being deemed not age appropriate. In many places The View From the Cherry Tree has been quietly been charged in and out of libraries with very little controversy. It's often found on summer reading lists.
In the above example, when Cherry Tree was challenged it was a supplemental reading text for the sixth grade. It was not required reading for the entire class. After receiving a complaint about the book the school principal referred the matter to the district office. The book was reviewed by a special committee. The committee recommended that the book remain in use. The parents appealed to the school board.
With one dissenting vote, the school board decided to retain the book for classroom use. "This is one book out of many," said board president Don Hewitt. "These days, kids can go home and read Stephen King or watch it on TV. If we were to allow this book to be taken out for the term S.O.B., next time we might have to take out ten books. That's not the American way."