Would you want them to sing it in front of teachers and friends?
I wouldn't WANT them to sing it.. if they do.. I don't care. I'm pretty sure their friends and teachers aren't so sheltered that they'll faint on the spot.
Xmas has almost nothing to do with Christianity anymore anyway. Santa is a ln invention from coca cola and the industry is milking it for all it's worth. It's just about presents and consuming goods ... So I honestly don't care so much if my kids say oh look there's a santa. He looks cute.. and all the sparkle and lights are nice. We don't celebrate it but it doesn't mean we have to pretend Xmas doesn't exist. My kids know that we have Chanukka and some of our neighbors have Xmas.