In the US the school calendar originally had a lot to do with the harvest. Kids were out in the summer to help their families on the farm. Also, recently northern schools have started doing winter breaks at Christmas time and then again a week in February (which makes perfect sense to me). It allows families to take a vacation at the worst time of the winter, and also saves on heating bills in the schools. Also, some counties work together to have different spring breaks. Like Palm Beach county and Broward county in FL border each other and one has spring break the week before easter, and the other the week after.
Back to the farming aspect. This can even be observed at the college level, not so much now, but years ago. I went to Michigan State University, which was originally Michigan Agricultural College, and even when I went 20 years ago they were still starting late September. They have no switched to the more common late August time frame.