https://phys.org/news/2018-05-student.html

The findings were clear: students scored lower when they had just experienced a hot school year than when they had just experience a cool school year; low-income and minority students were impacted by heat more than others; and air conditioning in schools all but completely eliminated the impact of heat.

"We show that only school-day exposure to higher temperatures affects test scores; hot summers and weekends have little impact on achievement and controlling for such exposure does not shrink the magnitude of impact of hot school days," the authors write. "This suggests that heat's disruption of instruction or homework time is responsible for the observed drop in test scores."