okay, I can see not celebrating christmas and/or not believing in santa - but I imagine that if this were my family (and actually, I am atheist - but we do celebrate in a non-religious way)
I would A) keep my kid home. It's one day. One freakin' day. Kids who homeschool don't go the whole year! If it were my DS, he wouldn't even realize that he was missing school, for all he knows it would be another holiday where school is closed or heck, I could tell him my car didn't start if I wanted to lie
or B) I would send my kid, and my kid, being a kid who doesn't celebrate christmas or do the whole santa thing could watch the other kids do whatever they do w/santa at school. Do they sit on his lap? I don't know - and honestly, that kinda creeps me out anyway - does he stand around saying merry christmas ringing a bell? does he sing a song, or read a story? Tomorrow, the 'santa' at my DS's school (btw, he and I have talked, and he knows it isn't a real santa visiting) is going to be set up in the pretend (he knows this, too) North Pole at the end of their polar express train ride and give each child a small gift (knowing his school, it will be a book and/or maybe a bell).
I honestly don't get why my kid couldn't either watch whatever santa at school does, or even participate. If my home values/religion were that strong - yet not such that I kept him home - why would seeing santa at school change anything? Do you keep your kids away from the mall? or from the entrance of stores where a santa looking dude is ringing a bell for donations?
I guess I just think that if it's such a big deal for some families due to religious reasons, they should keep their kid home or in the classroom, or suck it up and participate (I mean that it a nice way, like a pp said about traveling and other cultures). Here, in the US, the vast majority of people DO celebrate christmas, including the whole Santa Claus portion.
I don't really see the big whoop. If it bothered me, I would bring it up to my PTO and school admin and encourage the parents to have a vote in whether or not it happened in the upcoming years.