If it was a real problem, someone would have done it already, and it would be published somewhere. But it doesn't seem they have. You wouldn't have to fetch a rock, just give one of many links that would be available. But they aren't. Because it is not a real problem (i.e. porn or other such material available in school libraries).

I can imagine isolated incidents occur. Mistakes happen to the best of us. Or maybe a rogue teacher on occasion. But it is not a system-wide problem until someone can show me that it is.

It's been a few years, but I raised a daughter in the public school system. I was VERY involved, and contacted teachers and administrators on several occasions. There was no porn, or similar material, in the libraries or as part of the curriculum. My biggest issue was abstinence-only sex-ed. I actually took off work to sit in the back of those classes to see what they were telling her. It wasn't as bad as I feared, but I still needed to fill in some gaps for her. (She became the answer girl among her classmates...if she asked a question, I answered it, and apparently she passed it on to others. At least her classmates were getting straight information because of me.)