The First Amendment only says "Congress shall make no law"--it doesn't say anything about schools observing religious holidays. It's really not even binding on the states, as written, let alone local schools, it has just been extended by the courts, and relatively recently, at that.
This.
And if you wanted to interpret the 1A VERY BROADLY (and incorrectly) to define congress as any governmental law making body (e.g. a school), it still wouldn't apply, because the school's observance of a particular religious holiday is still not a rule that establishes a religion or prevents the free exercise there of.
And if we take it further, and interpret it to mean what the FFRF and like organizations would like it to mean, we would still have a problem. Forcing removal of all reference to religion in government by default forces the government to become atheist. The way I see it, atheism is a religion itself, so atheism becomes the government favored religion and thus violates this fictional separation of church and state.
But all of that is moot, because the 1A says "CONGRESS shall make no law..."