I spent my whole life in Arizona and graduated in the seventies. We often started in September. Sometimes we would have a day of school, then get Labor Day weekend. Yuma.
In Iowa we started the Thursday before Labor Day. Might even have been a 1/2 day on Thursday. So you eased into it. A half day, then a full day, then a 3 day weekend. Then a 4 day week. Then regular 5 day weeks.
We got 2 days for Thanksgiving.
Two weeks for Christmas.
Good Friday and possibly Monday after Easter.
That was it.
We were done around the 3rd of June, plus or minus.