It's a novel that begins with a school disaster and includes all kinds of violence, including sexual.

Deciding what books are appropriate for a high school library isn't an exact science, and it's often down to a few people carrying out a district selection process. It's not as simple as "you have to stock every book else you're guilty of censorship", regardless of the motives of people seeking to exclude (or include) specific books.

And for what it's worth, thousands of books are rejected each year by American school districts. Publishers, authors, and other interests push hard to get 'their' books into schools.