I agree with Kystar. Get a good look at your legal standings, and act accordingly. There is no tolerance in my books for that sort of prejudice. This is harassment on a "minority" religion (not really a minority, as we know, but many will not admit it to the Census) and you have rights.
I would try to keep the hissing and spitting to a minimum, but if you can have a long and calm talk with the "head teacher" about what right he has to dictate the religious upbringing of your child and embarrassing him in front of his class, all the better.
I would say maybe have a talk with your son about when to mention this stuff but that's absurd. The dark ages are over and darn it! We should not have to hide it just so these b***ers can pretend that they are right!
Maybe talk about the history of the persecution of our ways in history to your son, and how many people are still in that mindset, and it only means that they are insecure in their own religious standings (How could you be secure in believing in a hateful god?), that the Christian religion is expressing their way of honouring the Divine and sometimes it gets out of hand and they think that their way is the only way.
What has happened is not right and you need to take a stand. I am so sorry that your son has experienced this. Be sure that he will be a stronger person, and probably a stronger Pagan because of it.
Crazy Healer Lady
Health and happiness to you!
The purpose of a relationship is not to have another who might complete you, but to have another with whom you might share your completeness. -CWG