Young kids are very impressionable with this sort of thing.
Which is precisely the problem, as the school administrators (assuming they allowed sexual material in the library in the first place) should very well know.
Much like what you've said, kids at that age are too naive and trusting of authority figures to know when they're being conned or not, leading them to think that whatever's in the school library must be okay, seeing as the adults have already vetted it. Sadly, that's less and less the case nowadays — that is, assuming it hasn't been "quietly" at work for a while (which, sadly, it very well could've been).
When I was ten (I was an early bloomer, so-to-speak), I had my first "girlfriend" -- all I did was basically look at her a lot in class, causing her to cutely blush and smile, and we hung around each other constantly... for about a week before it "fizzled out".
I also remember feeling as confused as fuck and utterly lost as to why I felt different around her.
Remember having "Butterflies in my tummy!" at that age, too, yeah. Not much else came of it, though in my case, it was more like five or six (read: still in Kindergarten) than in grade school.
Feeling's been pretty "sporadic" since then, as while I've certainly come across girls who I thought were pretty, I've yet to have another major crush since some mild "puppy love" I had back in seventh grade.
Now, take crops of kids who feel as confused and as awkward as I felt back then as an innocent kid, and then introduce disgusting books like these to the library.
It isn't going to end and hasn't ended well. We're going to see more cases of kids "experimenting" with each other unless this madness is stopped.
Fucking groomers.
Have already said it elsewhere, but to repeat it here: Thank God that I finished elementary school almost a decade ago, so that the Groomers never got to me.
Of course, much of my luck is probably attributable to how it was a Catholic school, meaning that — while I came to dislike the religious teachings towards the end, hence me becoming an agnostic — I still got an above-average education, more trustworthy teachers, and a safer learning environment than what you'd expect from most American public schools. Upon reflection, that kind of upbringing probably saved me, so between repeating nine years in Catholic school and attending weekly Mass again, or spending those years in public school instead... yeah, I'll gladly take Catholic school and Mass that I'm only half-invested in, thanks.