There is a section in my local library listed as Adult Fiction. I think parents should be ultimately responsible for books their kids read and I'm not set on a rating system, just a way to note adult content. Actually, many libraries and bookstores do monitor this, just like they do music now.
In 7th grade, my son wrote a short story and included the "F" word. They were going to suspend him. At the time he was reading Jurassic Park. This momma walked into the principal's office with book in hand and threw a fit about suspending my kid. I pointed out the double standard. He had checked the book out of the school library and it has the "F" word on almost every page. I pointed this out and asked how they could let him read it but penalize him for writing it. BTW, without the word, it would have been an A???? Really? Suspension thrown in the trash. He also wrote a serialized short story for the school paper--9 parts, 9 issues. He used the word straddled for a person putting one leg on one side of the stick shift and the other leg on the other side so she could be close to her b/f. They made him take the correctly used word out and write "sat across."
So, Sol, just a way for us to note content is enough. That way we are not responsible for a misguided teen.