More than two dozen school districts across South Dakota have shut down early, with temperatures above 100 degrees.
Eureka Superintendent Bo Beck says "the safety and welfare of students and staff members" is a major factor.
His district and others dismissed a few hours early Wednesday because classrooms lack air conditioning.
South Dakota districts have called off classes before due to late-summer heat. But Beck says school closures are more common in winter months when snow, frigid temperatures and winds make travel unsafe.
A ridge of high pressure has made the northern and central Plains area the nation's hotspot.
The National Weather Service says high temperatures Wednesday were expected to range from the low 90s in northeastern South Dakota to as high as 107 in the center of the state.