The school year was different when I was a kid back in the 1940s and 50s. In California, which was proud of having the best schools in the nation, all public schools started the day after Labor Day and ended the second week of June. In between we attended classes Monday through Friday every week except for two days off at Thanksgiving, two weeks off for Christmas and New Year's Day, and single days off for Washington's birthday, Lincoln's birthday, and what is now called Memorial Day. We also had a week off at Easter.
In those days it was okay for schools to celebrate Christmas and Easter because they only talked about Santa Claus and the Easter Bunny, which weren't considered religious. The Jewish kids in our school were sometimes upset that Santa Claus never brought them presents, though.
It's interesting that now some public schools discuss religious holidays of many different faiths, but still only mention Santa and the bunny when they talk about the Christian holidays. Others avoid mentioning any of them.
School calendars have changed in many ways and now there are year-round schools, charter schools, and different school districts have different calendar schedules. And now many students are home-schooled, which used to be illegal in our state when I was a kid.