(CNN) -- A heat wave in parts of the Plains states and Midwest kept temperatures Monday as high as 20 degrees above normal, and it's likely to last until the end of the week.
Minneapolis set a record temperature for the day on Monday with 97 degrees, CNN meteorologist Dave Hennen said. The normal high is 79.
Heat warnings and advisories are in effect for eight states: North Dakota, South Dakota, Nebraska, Illinois, Wisconsin, Iowa, Minnesota and Missouri.
The heat index in Iowa reached 110 degrees Monday afternoon, Iowa's Department of Public Health said, and it warned residents of the risk of heat-related illnesses. It said even young and healthy people could be affected if they are active in hot weather.
The high temperatures are forcing a number of school districts across the region to make changes to their schedules.
Minneapolis Public Schools canceled all after-school activities and athletic practices Monday because of the heat, the district said.
Five elementary schools in Fargo, North Dakota, are closed through Wednesday because they are not air-conditioned, said Jeff Schatz, the superintendent of Fargo Public Schools. He said he was in one of the buildings at 6 a.m. Monday and the inside temperature was already close to 85 degrees.
The closures affect about 1,300 students in kindergarten through fifth grade, he said.
A malfunctioning air conditioner compressor in a Sioux Falls, South Dakota, elementary school forced teachers to dim the lights, close the blinds and use fans to keep things cool, said Sioux Falls School District spokeswoman DeeAnn Konrad.
Plans are in place to move children in the affected classrooms to an air-conditioned private school across the street, she said.
In hard-hit Iowa, schools in the central city of Marshalltown dismissed at midday Monday and planned to do the same Tuesday, school district spokesman Jason Staker said.
"Not all of our schools have buildingwide air conditioning," Staker told CNN. "When the temperatures outside reach into the high 90s like they did today, it doesn't take long for the temperature to rise in a classroom, especially when it's full of students."
The cause of the heat wave is unusually strong high pressure over the northern Plains and parts of the Midwest, Hennen said.
CNN's Mayra Cuevas contributed to this report.