Your question is not State Based although it is under the NY legal forum. Yes, there have been many conflict of laws as to religious freedom. For instance, the Supreme court premitted a religious group to do an elligal drug - i.e. Payote as part of their religious ceremony.
Your inquiry is based in constitutional precedence. Your elementary school does NOT have the Constitutional right to promote "THEIR" RELIGION. However, YOU and any other private individual has the right to invite another to "their" celebration. Of course, under our constitution you have the right to refuse to let your son attend.
Do you see the logic? THE CONSTITUTION IS PROTECTING Y O U! It is giving YOU THE RIGHT TO INVITE ANOTHER TO YOUR SERVICE AND RELIGIOUS INSTITUTION!!!!!!
Thanks for the question - cool - Vince