Each 1°F increase in school year temperature reduces the amount learned that year by one percent

From Heat and Learning by Joshua Goodman, Michael Hurwitz, Jisung Park, and Jonathan Smith.

From the Abstract:

We provide the first evidence that cumulative heat exposure inhibits cognitive skill development and that school air conditioning can mitigate this effect. Student fixed effects models using 10 million PSAT-takers show that hotter school days in the year prior to the test reduce learning, with extreme heat being particularly damaging and larger effects for low income and minority students. Weekend and summer heat has little impact and the effect is not explained by pollution or local economic shocks, suggesting heat directly reduces the productivity of learning inputs. New data providing the first measures of school-level air conditioning penetration across the US suggest such infrastructure almost entirely offsets these effects. Without air conditioning, each 1°F increase in school year temperature reduces the amount learned that year by one percent. Our estimates imply that the benefits of school air conditioning likely outweigh the costs in most of the US, particularly given future predicted climate change.

Hard learning in hot places.