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Air conditioning is common, but at least in some regions, it would be a tremendous expense for the the school to condition their buildings for occupation during the summer. And many buildings were designed around the summer break, so they may not have capacity to condition the buildings for occupation during the summer; this is not without its problems as some buildings end up being unfit for occupation during the school year, especially as the climate gets less consistent. There's probably some opportunity for savings in places where increasing hours during the summer could result in decreasing hours in the winter, though.

I think there's some cultural value in having a shared experience of summer vacation. But I agree, breaking up the breaks throughout the year, where possible, would make a lot of sense. There's a benefit of less crowding when school districts have different weeks off; although it's harder for extended families to meet up when their school schedules are drastically different.