D.P.
Ask around to some other moms who also have older kids in the school. My only son is in Kindergarten this year and I hate that everyone seems to think that we've all done this stuff a million times before! LOL
Don't forget, though, that "dressed up" means different things to different people. I wouldn't go over-the-top (frilly Christmas dress). Have you ever gone to an event where you are over-dressed? Nothing worse. Ugh.
I don't think you could go wrong with a simple white/green or red sweater/black pants or skirt and maybe a Christmas-y hair bow/barrette/or necklace/pin to Jazz it up a bit.