You still get the day off for Christian and Jewish holidays they just don't call them religious holidays.The conservative Education Action Group reported that the Montgomery County School District “stripped Christmas and Jewish holy holidays from its official calendar after Muslim parents complained,” and the headline warned that a “suburban DC school district takes Christmas off (the) calendar after Muslims complain.”
School board yanks Christmas from calendar after Muslims ask for their own holiday off
They give the Muslim students excused absences for the Muslim holiday of Eid ul Fitr if they want to miss school that day. The Muslims don't like that though they want the school closed even though there's not a lot of Muslims in the community.
I think it would be silly to close the school for a hand full of students. However if your closing it for some religions wouldn't it be illegal not to recognize other religions?