You understand this is a school library that they are talking about, where some of the children are 11 and 12?
School librarians get to decide and there are many places where they can (and should) find age ratings for books. Peters books, common sense media and there are loads of other sites that give trigger warnings.
someone else has mentioned grey areas, another has mentioned that some books are studied in school. I think people are missing the point that a library book goes home with the child where hopefully it is read, most likely alone. A book they are studying is read in class with a teacher who will guide the kids through any difficult issues. For example the racist language used throughout Sawbones might be hurtful for some kids and the context might need to be explained.
I think some people still believe that a school librarian’s job is just sitting about reading books, using a date stamp and chasing up returns. There is so much more to the profession and we have a duty of care for the kids in our care for 6-7 hours a day.