Speaking from personal experience of having watched Schindler's List in a high-school history class, it certainly is possible to see 'R' rated films in US schools if the teacher wants to show them. Not sure if that's specific to the state (VA) though and I have to believe it would be shown more if it was PG-13.

(incidentally, I'm glad we watched it and I think it was important to give perspective but I can see how some parents might have objected considering the brutality of the subject matter. I still remember the shock of seeing that red coat in that pile of bodies...)