If I were you, I would tell them to ask the administration. I really think that this is the safest thing for you to do.
Presumably, you were not part of the decision as to which decorations should appear in the lobby. Therefore, you do not really know what the reasoning was (if any) for choosing those decorations. That means that you probably shouldn't try to speculate. In addition, this can be a very touchy issue and can even at times lead to litigation. I do not think that you would want your words to be used in court, with the administration wondering why you took it upon yourself to explain school policy.
I would definitely refer this one to the principal or, at the very least, ask him/her what you should say.