My schools (elementary, middle, high) were always polling places when I was growing up.
The school next to my apartment complex has also been a polling place, at least for the past 13 years I've lived here.
School was in session when I was a student; school was in session this year as well -- the voting adults and school kids able to coexist and go about their business in the same place, at least in 1960s-'70s Milwaukee and present-day San Diego. Adjustments had to be made for either phys ed or theater classes (if the gym or theater/auditorium was in use.) -- but a day off from school? Pffffft!