We've had that debate and if you are a regular reader you know that I am fine with Christmas trees in classrooms. They have crossed some undefined boundary to me from religious symbol to secular cultural symbol. But, not everyone agrees. Thus, the key in these instances is to know your communities. Obviously, Ames being a liberal college town is going to have less tolerance for Christian symbols during the holidays. Some rural Kentucky towns around me want those symbols, so it is all relative for you school administrator types out there.
So, while we lawyers fight over secularity, the Lemon test, etc ... just do what you think is best by your community and most of you should have a fine (and quiet) holiday season.