My wife and I have decided that it is most important that our children learn that it is important to respect the adults put over them, even when we disagree with the adults.
This teacher created a dillema for your son by changing the rules after the fact. I bet the teacher did not specify what was proper in the thank you note until after your son wrote what he wrote.
I think the best most respectful posistion your son should take is to withdraw his note. This path maintains respect for the teacher while allowing your son to maintain his stand.
Thank you for the advice. I think that a slight rewording that instead invites the teacher back (he didn't know this was an option when he wrote the original note) to present information about the slavery, torture, and killings done in the name of Islam would be a good way too. Not only does my son get to register his protest in a more tactful way, but he gets to achieve a larger goal of helping his classmates understand the issues that have not been presented to them previously. I'm leaning in that direction, but I have to get more information from my wife first and also from my son to make sure that I fully understand the situation and his feelings about it.