Wrong line of reasoning. This is an issue of discrimination, not the first amendment. Kids do not have first amendment rights so long as the school is acting
in loco parentis. The school is responsible for the children's safety, and can do whatever it sees fit to provide a safe environment, including confiscating notes, censoring messages on clothing, enforcing dress codes, whatever.
However, in an age when atheism, Islam and Kwanzaa are imposed, it's outrageous to prevent children from sharing their social heritage.