I say organize the parents and kids to bring all of the Christmas paraphernalia they like, and just dare the schools to do something about it.
May I suggest a devilish variation on your excellent idea? It is, quite simply, EVERY DAY IS A HOLIDAY (HOLY DAY, if we dare). It would drive the atheists, secular humanists, postmodern liberals and their diverse coalition of related crackpots, even more insane than they already are. It would drive home the point, 24 hours a day and 7 days a week, that these "sensitive" multiculturalists are, in fact, ANTI-cultural and INSENSITIVE. It would challenge them with the truth that the worst kind of fundamentalism ever to insert its cancerous tentacles into the American body politic is the leftist fundamentalism known as political correctness.
To quote a Catholic saint, Teresa of Avila, "Live today as if it were the last day of your life, and you know you are about to face judgment."