There's two common "Christian Holidays"
Easter and Christmas.
Since schools aren't generally in session on Sundays, easter isn't given off. Good Friday sometimes is, but I've never known it to be given off FOR Good Friday in "respect to the Christian Religion". It's usually given under some other name.
Then you have Christmas. While most schools seem to have gone to "Winter Break", some seemingly do still potentially do Christmas.
However the issue with Christmas is strangely the same issue with marriage.
Christmas is both a Religion and Secular entity at this point in our society.
There is "Christmas" the private holiday potentially celebrated by Christians concerning the birth of Jesus Christ.
Then there is "Christmas" the official Federal Holiday, which is secular in nature and does not directly require a "religious" element anymore than "Marriage" does.
The argument claiming that religious arguments are irrelevant in the "Marriage" debate because when talking about the term in relation to the government it doesn't involve religion equally applies to Christmas. When talking about it from the statement of a federally recognized holiday, it doesn't involve religion.