For the record, I am not anywhere near as knowledgeable as I should be on this topic or the redistricting discussion. Especially with a son about to enter kindergarten this fall.
Are all school buildings air conditioned? Even so, I would suspect the great majority are not at all energy efficient and year-round school would introduce them to huge utility bills/cooling challenges. As for sports/activities holding back the change...you can add the Travel Federation of Iowa (TFI) to that list as well.
I attended a statewide CVB/DMO meeting in Des Moines earlier this month where schools were the topic of one discussion. Specifically, school start dates. Many of our larger statewide tourist attractions have expressed concern for many years about the impact of early school start dates on the Iowa economy. E.g., Iowa State Fair, Lake Okoboji, Adventureland, Amana Colonies. The concern is that early start dates impact attendance at these very large family-oriented attractors. Attractors that deposit millions of dollars into Iowa's economy each and every year.
Anyway, Michigan addressed this issue a few years back and it is now state law that school cannot start before Labor Day. It appears most schools adjusted to this by adding 12-15 minutes onto each and every school day. Here is a story on it: http://www.savealabamasummers.org/states/michigan/bill_requires.html
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Disclaimer: As much as I publicize our community as being the literary capital of the country, I am a terrible writer. Please excuse my typos, run-on sentences, and serious ellipsis addiction.