I remember studying the book In the Heat of the Night in Grade 8 English class, and the teachers decided to show us the movie as a treat. (Very good - starring Sidney Poitier and Rod Steiger, who won an Oscar.) Trouble is, the teachers probably didn't realize that there was a mostly-topless scene for one of the female characters. No-one got into trouble, as far as I know.
Fifty Shades, though? Yeah, you'd have to be completely out of the loop to not realize that it wasn't suitable for a classroom viewing.
Even if she honestly had never heard of the movie, what adult in his/her right mind would let the students pick the movie and not bother to even check to see what the movie is about? Also, isn't the rating *on* the DVD itself? I'm thinking if the students handed her the DVD without the DVD case and the teacher didn't have internet access in the classroom, she still would've known an R-rated movie is already inappropriate (or, at best, questionable) for minor students.