Yeah, the windows would open. But, honestly some days were gross. But US school would end by mid June and start in September so there’s only a few hot days to worry about, you might hit over 80F /27c a few times at most.
I can’t see anywhere which would explain why the school closed. It’s possible that it doesn’t have central air. It could also be an efficiency thing; maybe the AC can’t vent out the heat fast enough to cool down the building when it’s that warm? My house is actually like that when it hits around 105° outside.
The smart ass reply is they close down when it’s hot for the same reason Texas closes down when it’s only 30° and it barely snows 😅