J.S.
Maybe I'll be in the minority here, but why not try just looking at it from a mellow point of view wherein the teacher or school figures Christmas gets so much exposure that they thought they'd balance it out by offering some educational info on other cultural and religious winter practices? Sounds like your child is a part of a diverse classroom, which is a good thing. Maybe you would like to volunteer to be the parent who comes in to talk about Christmas?
Peaceful holiday wishes