I agree as someone who is a fan of some of these series. Even BNHA had some darker themes in the later chapters with some relatively devastating wounds, and death. I agree that the specifics of the grades in which this is happening is key, because in the past there were cases of very young students having access to series like Maid Dragon which is 100% an adult series.

I want to add another consideration. I got into anime around the end of middle school, thanks to Toonami and accidentally having the VHS tape record into anime hour on Adult Swim. After that I was easily able to find anime, manga, and doujinshi on the internet. Any kid with a degree of intelligence is easily going to be able to find manga online. For those who can't, and are interested they are completely free to discuss it with their parents when at the book store (although my Barnes and Noble has manga in the adult section for a reason).

I never really viewed the school library as a source of entertainment. In college you could of course find entertainment but that was because our library was available to the public. In grade school however, the only time I went to the library was to get reference material for a paper.

I'm an advocate of parents being vigilant of what their kids watch/read. It makes it a little easier on parents if kids have to go to them for entertainment sources like manga instead of being able to just go and rent Tokyo Ghoul, Jujutsu Kaisen, or Shingeki no Kyojin (If you have seen these or read them, and you would let a child read them...you're wild for that).