I agree that informing the teacher of your feelings and convictions is a good idea. However, the delivery needs to be constructive and not abusive or aggressive. Calmly make your point and let it be. But I think it's more important for you to talk to your son and explain why this is inappropriate. Just because they may have already been exposed to this or worse on the internet, does not negate your responsibility as a parent to teach your son right from wrong, moral from immoral. Training of such things should start at home and continue on to adulthood.