Back in the 1960s when I was attending NYC public schools, Yom Kippur was a day off due to so many
teachers being Jewish and taking the day off anyway.
I wonder if that's a factor here, rather than just general multiculturalism?
Probably. Why keep the school open if no one is going to show up?
A district near me tried to do away with Good Friday. Teachers and students said they wouldn't be in that day. They closed the schools.
Back in the 1960s when I was attending NYC public schools, Yom Kippur was a day off due to so many teachers being Jewish and taking the day off anyway. That's the precise reason. That's also why schools don't give off for major (but less observed) Jewish holidays like Sukkoth, Pesach, and Shavouth. It's not the religious significane of the day-- but the prevelance of it's observance. Those same teachers who would skip on Yom Kippur wouldn't skip class for Pesach.