Ms. Washington ignored the text — she thought the school should be taking advice from public health experts, not parents. But other parents flooded the board with messages, and the school stayed open. Within a week, the number of cases had nearly quadrupled. A teacher was hospitalized and put on a ventilator. When the board finally closed the school temporarily, 77 students and staff members, including Ms. Washington’s daughter, had tested positive.

“We’re talking 30 days in, and it went completely out of control,” Ms. Washington said.

Her daughter’s school, Corner Canyon High School, experienced one of the biggest coronavirus outbreaks at a school in Utah, and possibly the country, with 90 cases within two weeks — most likely an undercount, since not all students and staff who were exposed or symptomatic got tested.

And Corner Canyon was not the only school in the district to have an outbreak. By Sept. 28, Canyons School District, with roughly 33,000 students, had temporarily closed three high schools and a middle school, telling about 8,000 students to learn from home.