Inappropriate according to the school. Schools determine all sorts of things along those lines. They choose what to teach in the first place. Is that in violation of freedom of speech too? School libraries aren't unlimited. They can't contain every possible book. And you wouldn't want them to contain, for example, Playboy magazine or other actual pornographical publications. Schools are obviously allowed to determine what is and isn't appropriate; this is much better, having it decided locally, rather than what, deciding at a national level what is and isn't okay.