If a student can go to a theater and watch a rated R movie, then there is no reason they can't watch one at school. However if they are in a class with underclassmen the underclassmen should be required to sign a permission slip of some sort, allowing them to see the movie. If they don't get it signed, or don't want to watch the movie (including 17 year olds) then they should be given the option of an alternate assignment.