We understand that public school prayer discriminates against some religious views so it is prohibited in public schools. Similarly, Christmas concerts play a role similar to the teaching of creationism and prayer. The Christmas concerts subconsiously influence students toward the beliefs of Christianity. One could recognize the beliefs of many religions or none. One could play music from several religions or non-religious music.
Religion is a personal belief.
There are so many religions to choose from, including the choice of no religion. It is impossible to decide that one belief is right and another is wrong. So it is reasonable to say that it is unconstitutional for government to favor Christianity over other religions, including Athieism. Instead of using tax dollars to decorate the streets for the holidays, we could use the money for other things like playgrounds and helping the homeless. Also, students could play music that has no religious meaning to please every belief or offend none.