IIRC, U.S. school schedules initially developed to revolve around the needs of agriculture, when a lot of kids had to help out on the family farm in the summer months.
We have had year round schedules in California since the 90’s. Not sure about the rest of the country.
Is that everywhere in California?
No idea, and as moviefreakedmind pointed out, the traditional summer break model holds sway in farming areas. Lots of farming out here too.