I have never ever seen smutty novels going around a class.
Anyways, to step away from who is the most permissive parent, there is a problem providing good literature to teens. The 'written for' teens (eg Anthony Horowitz) can be very formulaic and limited in vocabulary. Adult books can contain unsavoury side stories.
What we do at our school is donate our used (clean) adult books to the school library, so that the students have a decent selection of varied books to choose from, in addition to the typical teen market books.
One of the problems I have seen as a parent of 4 teens, is that if you leave it totally up to them, they will choose the same genre of book (sometimes the same author) over and over. You may think they are reading well, but they are not doing an awful lot to extend their vocabulary or exposure to different writing styles.
A good school librarian is worth their weight in gold. Our librarian works with each student individually to recommend books, and also to prise book reviews out of willing volunteers.
As for the topic of the OP, I know that when I was at school, I read all of our texts in the summer holidays. We bought our own books at the end of the previous academic year, so we had them in our possession. I also tried to catch a production during the Edinburgh Fringe. I don't think reading Pride and Prejudice for yourself is even remotely the same as going through it in class, but it is good to have an overview of the story before the teacher reveals all the deep, hidden meaning.
DD is doing P&P for her GCSE, and they have been instructed to read the book before they start covering it in class. I have also bought her the BBC DVD (Colin Firth) for Christmas, and already have the Hollywood version from a few years ago.