On a Friday in mid-September, Sunny Washington got a text from another mother at her daughter’s high school in an affluent suburb of Salt Lake City. Three weeks into the school year, the number of coronavirus cases at the school was rising, and the district was considering shifting to online instruction. The text urged parents to beg the school board to keep classrooms open.

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 Washington’s daughter, had tested positive.

“We’re talking 30 days in, and it went completely out of control,” 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 about 33,000 students, had temporarily closed three high schools and a middle school, telling about 8,000 students to learn from home.

The story of Canyons is an object lesson in what can happen when schools reopen in communities that are failing to contain the virus. In the two weeks before the district reopened, Salt Lake County had roughly 187 new cases per 100,000 people, a level at which some experts have advised against high schools opening in person; that level is 2 1/2 times higher than the standard Washington state uses to recommend distance learning for all students.

Since then, with schools and colleges open, things have only worsened, as both the county and Utah have become hot spots. In the two weeks that ended Thursday, Salt Lake County had nearly 617 cases per 100,000 people. Over the last week, Utah had the sixth-highest rate of new cases per 100,000 people of any state and set a state record for the number of people hospitalized with the virus.

Last week, Gov. Gary Herbert ordered stricter restrictions on social gatherings and mask wearing in Salt Lake County and other hard-hit counties, calling Utah’s outbreak “one of the worst” in the country and “unacceptable.”

Yet despite the conditions at the start of the school year, Canyons School District took only modest steps to prevent the virus from spreading in its buildings.

The state had mandated that students and staff members wear masks in school, allowing unmasked sports practice and competition. But with nearly 80% of Canyons students opting for in-person school, the district seems to have made few adjustments to accommodate social distancing in classrooms. The district also did not make coronavirus testing part of its reopening plan, leaving those decisions to families.

Canyons is also an example of how the nation’s 13,000 school districts are struggling to find workable policies in the absence of clear standards from the federal government and many state governments. The result has been a patchwork of policies varying from state to state and often district to district.

The Centers for Disease Control and Prevention, which has faced pressure from the White House to avoid discouraging school districts from reopening, has declined to provide specific guidance on what districts should do when infections rise in a school or the surrounding community.

The Utah Department of Health advised switching to remote learning for two weeks if a school reached 15 cases within two weeks or, for a very small school, if the number of cases represented a tenth of the student population. But the guidance was only a recommendation.

“We’ve forced every school district to figure out how to respond to a pandemic on its own, and it’s insane,” said Dr. Ashish Jha, dean of the Brown University School of Public Health.

“There should be clear guidance — whether it’s Department of Education, or CDC, or ideally a combination — so that you don’t have every school district in America with different thresholds, different approaches, different measures.”

Over the summer, Canyons said it would adhere to the state Health Department’s standard for closing schools. But when Corner Canyon reached 15 cases the school board decided to ignore the guidance, shifting the school to a hybrid schedule instead of going fully remote.

The board ultimately adopted its own standard, which stipulated that it would shift high schools to remote learning when positive cases represented 2% of the students attending in-person classes — mostly, one board member suggested, because at that point the number of students quarantined from possible exposure would become unmanageable.

When the board decided to close Corner Canyon, on Sept. 18, the school’s 77 reported cases represented more than 3% of the roughly 2,250 students attending in person. (By contrast, New York City, which recently reopened schools on a hybrid model, has said that in certain situations it will close schools for only two positive cases in separate classrooms.)

In a Canyons board meeting Sept. 15, when Corner Canyon was at 42 cases, one board member, Steve Wrigley, said he had looked in vain for national standards. “There really is not many guidelines out there right now — everybody is sort of flying by their seat,” Wrigley said.

Another board member, Clareen Arnold, cited a CDC statement about the importance of in-person school to children’s mental health and development, inserted at the behest of the White House, as an argument for keeping Corner Canyon open.

In a community that parents and teachers described as deeply divided over whether the virus represents a real threat, the board’s decision left parents on both sides angry. Some were upset that the board had ignored the Health Department’s guidance, while others thought that schools should not close until 10% of the students had tested positive.

Before the board made its decision, parents and students gathered with signs saying things like “Keep Our Schools OPEN!!! Keep Utah FREE!!!” One mother argued that since no students were being forced to attend school in person, no students should be forced to stay home.

Many parents in the district do not support any virus restrictions. After Corner Canyon canceled most homecoming events, some parents organized their own homecoming party. The mayor of Draper, where the school is, said that he wished his constituents would follow public health guidance on matters like masking and social distancing but that he could not force them to.

“I don’t think people are going to respond until they see people go into the hospital,” Mayor Troy Walker said.

Walker said he had heard from some Corner Canyon parents that there was an agreement among mothers at the school — he called it a “mom code” — not to get their children tested for the virus even if they became ill, to avoid adding to the school’s case count and contributing to it being shut down. (He said he told these parents he did not agree with this approach.)

Many parents and teachers are still bitter that the board did not close schools sooner. “We feel like we were deceived,” said Katie Nelson, a special education math teacher at a middle school in the district.

(Lindsay D'Addato | The New York Times) Jennifer Santos, who complained when her son's high school remained open despite 19 coronavirus cases, in Draper, Utah, Sept. 26, 2020. In the suburban Salt Lake City district, coronavirus cases spiked as students returned to their classrooms.

Jennifer Santos, whose older son is a freshman at Brighton High School, another of the schools that temporarily closed, said that when the school reached 19 cases and remained open, she complained on the district’s Facebook page.

“The answer was, ‘You’re welcome to keep your child home,’ ” she said.

Other parents, however, said that they believed that the academic and mental health benefits of being in school outweighed what they saw as the minimal risk posed by the virus.

A board member, Amanda Oaks, said that while there was concern nationally about the risks of students or teachers becoming ill from coronavirus in school, “My honest fear and the fear of some of my fellow board members is that that could completely flip the other direction as soon as we get a teen suicide associated with quarantine isolation.”

Some teachers in Canyons also feel strongly about the value of keeping schools open.

The teacher who was hospitalized, Charri Jensen, who teaches sewing and design, recovered enough to go home. In an interview, she said that she wanted people to take the virus more seriously. But she also said that when she was well enough she planned to go back to work.

She had become a high school teacher because she loved the social rituals of high school — “the dances and the football games and the assemblies and the extracurricular things” — and it made her sad, she said, that her students were missing out on some of those traditions.

“There are these things I want these kids to be able to experience in life,” she said. “But then, is it worth it — for life, you know?”

The increase in cases, driven by 15-to-24-year-olds, began in early September, shortly after schools reopened and students returned to colleges. The state Health Department believes the surge started among college-age adults in Utah County, just south of Salt Lake County, home to the state’s two biggest universities, and then spread to high school students. Walker, the Draper mayor, thinks that some teenagers in his town were infected when older siblings came home from college for the weekend.

Since the semester started, a dozen schools in Salt Lake County have temporarily shifted to online learning because of high numbers of cases.

In September, as the Canyons board put off closing Corner Canyon High School, district officials and board members said that a vast majority of cases in the district’s schools were the result of exposures outside of school and that there was minimal spread within schools themselves.

But a spokesman for the Salt Lake County Health Department, Nicholas Rupp, said it was very difficult to determine in most cases where someone was infected.

In any event, once Corner Canyon shut its doors, cases among students and staff fell sharply. After a month of being closed, the school is set to reopen Monday. As of Wednesday, according to the district’s dashboard, it had between one and five cases.