Is it? Just because it's inappropriate for one age group at a school doesn't mean it's inappropriate for all age groups.

You can generally categorize the age groups present in a school by... looking at the grades served in the school. shocker.

I'm not entirely sure what this is supposed to mean. Like... yes? But my point was, a school may have a very wide range of age groups. A K-12 school will have students from ages ~4 to ~18. Even a regular primary school will have 7 year olds along with 14 year olds. Point being, a book that is inappropriate for the younger students at a school may not be for the older students; and I don't see why the older students shouldn't have access to those books.

Books are already made available to different age groups in different ways. Libraries that serve different ages put different books on different shelves. There are books on shelves young kids can’t reach, and there are books behind the counter.