It doesn't matter that the US was not founded as a Christian nation. I'm not advocating that the US be a nation that only recognizes Christianity.
However what I do believe in this instance is that by calling it a Christmas concert the only issue is that someone's feelings get hurt.
Let's put it in another context. Do you, as a citizen, get to tell another citizen that their right of free speech or freedom to assemble is void because you feel that what they are saying or why they are gathering is against what you believe?
No, you do not. Therefore you do not get to tell my students, who call it a Christmas concert, that we cannot call it a Christmas concert.