Dude, public school's aren't Christian schools. Accordingly, the decisions about which days to give off must be dictated by practicality. I went to a public high school that was about 30% Jewish, both students and teachers. The school couldn't function if 1/3 children and teachers didn't show up on Yom Kippur and Rosh HaShana, so they closed the school. That simple. For the same reason, even the Friends school (which, despite its affiliation, was also about 30% Jewish) in my neighborhood gave off for Yom Kippur. Of course, all these schools gave off for major Christian holidays too.
Conversely, the Catholic school with basically zero Jewish students/teachers didn't give off for Jewish holidays, and the Jewish school, with zero Christian students/teachers, didn't give off for Christian holidays.
Similarly, if a school in NJ or Detroit has a large Muslim population, it's inefficient to hold school on Eid because a lot of people won't show up. Not only will the school have a hell of a time finding enough substitute teachers, but lessons learned that day to the Christian/Jewish students will basically have to be retaught the day after.