A blast of late summer heat caused disruptions Wednesday for schools from Michigan to Virginia, with some districts dismissing students early and others holding classes online just days into the new academic year.

While temperatures weren’t as high as last month’s triple-digit deadly heat wave, schools in states including Michigan, Maryland, Pennsylvania and New Jersey cited inadequate air conditioning in cutting days short. One Massachusetts district canceled two days of classes because of hot classrooms. Temperatures in the mid-90s also led to online learning in Pittsburgh and Baltimore.

Only 20% of public schools in Detroit, where the temperature reached 89 degrees Tuesday but dropped Wednesday, have air conditioning. The district dismissed its roughly 54,000 students three hours early Wednesday for the second consecutive day.

"We never want to inconvenience our families with early releases, but we also do not want our staff and students to be so uncomfortable that teaching and learning becomes a distraction to the heat," Detroit Public Schools Community District spokeswoman Chrystal Wilson said in a statement.

The early dismissals caused headaches for families who had to scramble to make last-minute schedule changes.

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Parent Natesha Myers, who works from home, opted to keep her 5-year-old daughter with her. Myers said she would not have been able to pick her daughter up from her Detroit school three hours early because of scheduled work meetings.

"It was very difficult and stressful," Myers said. "I had to give her the iPad. She kept trying to climb on my lap."

Late summer heat isn’t unusual. But temperatures at the start of the school year have been getting warmer for years.

For instance, Philadelphia's expected high of 95 on Wednesday is 13 degrees higher than the normal high for the day, according to National Oceanic and Atmospheric Administration data. The holiday weekend also followed the hottest August scientists have ever recorded with modern equipment; scientists blame human-caused climate change.

The first week of school in Philadelphia this week saw early dismissals for dozens of schools "without air conditioning or inadequate cooling." The district announced Wednesday that more than 80 schools would end classes early the rest of the week.

District spokeswoman Monique Braxton said many schools need upgraded electrical systems to support air conditioning.

"We’re in an old city," she said. "Most of our buildings are old facilities. We’re making adjustments as necessary."

In Baltimore, where temperatures climbed to the upper 90s, inadequate heating and air conditioning systems have also long been a problem.

Officials released a plan in 2017 to make all necessary improvements and repairs within roughly five years. While that deadline has been pushed back for issues including the expense, the number of city schools without air conditioning has decreased from 75 to 11, according to district officials.

Nationwide, an estimated 36,000 schools need to update or install HVAC systems, according to a U.S. Government Accountability Office report in 2020.

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On Wednesday, Baltimore schools without air conditioning dismissed some students early and assigned others to virtual learning for the rest of the week.

In Pittsburgh, students and staff in nearly 40 schools also pivoted to remote learning.

Returning to online learning in times of extreme weather — from hurricanes to water crises — has become more common after the pandemic, though remote instruction has long-term shortcomings.

In Lowell, Massachusetts, a community of roughly 115,000 about 30 miles from Boston, district officials said schools would be closed Thursday and Friday, with many classrooms "expected to be too hot for teachers to teach effectively." Buckingham County Public Schools, a rural Virginia county about 75 miles west of Richmond, dismissed students early two days in a row as "a precautionary measure."

Health experts warn that exposure to excessive heat can cause dehydration or heat exhaustion, among other things, while teachers say sweltering classrooms make it difficult to do their jobs.

"Teachers are concerned about the environment being conducive to education. They were thankful to have the relief," said Detroit Federation of Teachers President Lakia Wilson-Lumpkins said of the early dismissals.