On the last day of the 2025–26 school year this June, dozens of Philadelphia schools were released early.

Not for a snow day, but for the heat. About 52 district schools, which is nearly a quarter of Philadelphia's school buildings, still don't have adequate cooling. When a heat wave rolls in, the district sends those students home to learn remotely. If the temperature inside the building hits 90 degrees, school goes virtual.

Philadelphia isn't alone. Nearly half of U.S. public schools now open before mid-August, and roughly 70% of students are back before September. That means millions of kids returning to un-air-conditioned classrooms during some of the hottest weeks of the year.

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Should the school calendar be built around the weather?

The popular story is that summer break exists so kids could bring in the harvest. While it sounds reasonable, it is mostly a myth.

Farm kids were needed for spring planting and fall harvest, not the dog days of July.

The real reason schools emptied out in summer was health: Back in the early 1900s, long before air conditioning, packed and sweltering classrooms were dangerous. So weather did shape the school calendar a century ago, just not the way most of us think it did.

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Now the case for revisiting the issue is building all over again.

MORE: America's deadliest weather isn't tornadoes, hurricanes, flooding or lightning — it's heat)

Heat waves are arriving earlier and lingering longer, right as the first bell rings.

Denver has been calling "heat days" and releasing students early for years, because the average district building there was built 56 years ago, when air conditioning wasn't standard. As the district's facilities director put it, an early release is "our last resort,” and a real disruption to learning.

The cost of AC in schools

Outfitting the older buildings with air conditioning is staggeringly expensive. A new AC system can run $5 million for an elementary school and up to $15 million for a high school, and that's before the plumbing and insulation work, which can add up to $1 million more per building. Denver alone earmarked $240 million of a 2024 bond just to cool its last 29 schools. 

Many would argue that a later start date, by comparison, costs nothing. But it's not that simple. Testing windows, teacher contracts, sports seasons and family vacations are all built around the calendar we've got.

Some states have even written start-date limits into law to protect tourism. Virginia's "Kings Dominion Law" long blocked pre-Labor Day starts, and Michigan still requires them, though most districts now waive it. 

For now, most districts will continue to patch the problem: early dismissals, remote days, indoor recess, extra water breaks. But as summer keeps creeping deeper into the school year, don't be surprised if more districts start asking the bigger question on whether the school calendar needs to change.