If I were a high school English teacher, I would focus my plans on newer works of literature. So, for the sake of an example, say I assign My Sister's Keeper by Jodi Picoult. Well first, there would probably be no males taking my class, but more importantly, there would be a vast difference of knowledge between the readers of the book and the watchers of the movie. The 10 page paper that would be due for my class (now there are no males in the class, and half of the women just dropped), would definitely be an F for the person who writes about the death of the wrong sister. Don't ask me why the script writers decided to kill off the totally opposite sister in the movie than in the book, but this would be a detrimental fact that would be easily overlooked (or go unknown) to a "movie watcher" student.
That is just one example of why one should choose the book. I could go on with the other 14-23 examples I have in my head of why to choose the book, but only one will suffice for this posting. The amount of detail and work that goes into a novel can (sometimes) be a great deal more then in the movie, and skipping the text is simple deprivation. Especially during discussions of whether or not it was Anna or Kate that died. Do you really want to get an F?