Almost 100 million people across the United States spent the first day of summer on Thursday sweltering in temperatures that topped 90 degrees, as meteorologists warned that the high-pressure system that scorched the country for the past four days would linger through the weekend in many places.

The heat shattered temperature records and altered daily routines from the Midwest river valleys to the pine forests of New England, and left roughly one-third of Americans under extreme heat advisories, warnings or watches on Thursday, according to the National Integrated Heat Health Information System.

In rural Indiana, sheriff’s deputies conducted wellness checks on older residents. In Maine, officials urged homeless people to make use of cooling centers. And in the New York region, students left school early — some because their schools closed at midday, and others because worried parents took matters into their own hands.

Simone Machado pulled her son Bryan, 10, out of school at Ann Street Elementary School in Newark, the largest city in New Jersey, early on Thursday because of the heat. By the time she got there, a bright red heat rash had already bloomed across his neck.

His fourth-grade classroom was “very, very hot,” Bryan said. He was afraid to go back to school on Friday, when temperatures in Newark are forecast to reach almost 100 degrees, but at least it will be the last day of school before summer vacation.

“I don’t want to go. The rashes are going to get worse and worse,” he said. “School’s over tomorrow, thankfully.”