There's a big difference between showing something to a person and that thing being available in a library whey they can seek it out or encounter it at random. You seem to be implying that library curators are leading minors to shelves and telling them to look at something they otherwise wouldn't.
They may not be actively showing this material, but at the very least they are facilitating porn use by minors. The same way as when a person offers to buy a minor alcohol, they are facilitating alcohol consumption by minors.
There is no acceptable reason to keep pornography in a school library.
You think it's porn, many other people do not. I grew up in a hyper-conservative milieu where commercial porn was simply not available, everything I learned about sex came from books. I discovered the existence of sexuality through reading the Bible; I literally had no idea there was anything more complicated to life than moms and dads until I I happened across all these stories of people being killed for engaging in 'abominations'.
Magnetic poetry can 'facilitate porn use by minors'. I think you work toward a better-defined standard that doesn't pre-empt the rights of other people who don't share your point of view.