Many (but not all) private schools in the US are religious. From elementary school through college I attended Catholic affiliated schools. This sort of display would not be allowed at most public schools, and the ones that would allow it would be sued.
I’m lucky enough to work for an organization which furnishes me with up to 4 hours of paid leave to vote. Plus, my polling place is on the road home, and I’ve never waited longer than a couple of minutes to vote. Finally, doing it in person feels more impactful, even if that’s just a perception thing.