If anyone complains about the Christmas tree in school you might want to point out that it is an ancient practice whose origins are unrelated to Christianity. That a particular religion has tried to take it over as theirs should not be grounds for its prohibition.
If worse comes to worst, perhaps it can be salvaged by putting up a display called "Winter Solstice Celebrations in History."
There is a fine line between government promotion of religion and government suppression of it, with much argument over exactly where to draw that line.