I'm trying to put myself in the shoes of a public school administrator. On the one hand, banning Halloween as a Christian religious holiday would be almost as gratifying as ruining another traditional holiday with pointless controversy. On the other hand, it might anger pagans, who, like Muslims, Buddhists, et al, practice a lefty-approved religion. Not sure how I'd handle that one.
Banning Thanksgiving, on the utterly ludicrous theory that it has something to do with Native American suffering or European wrong-doing - that would be an obvious win.
Banning Columbus Day is even easier. Just so long as no one points out that Washington owned slaves and MLK had chicks on the side, and some Veterans have done bad things, as well. Just as long as no one actually thinks about what these holidays are actually meant to celebrate, I'd be good to go.