A growing number of Minnesota school districts are abruptly changing their back-to-school strategies, scrapping plans for in-person classes and opting to start the school year with distance learning.

The decisions, coming weeks — or days — before the start of the academic year, have been driven by worries over staffing shortages and fluctuating numbers of COVID-19 cases across the state, among other concerns.

In some cases, so many teachers and staff members have requested medical leave or health-related accommodations that school leaders weren't sure they would have enough teachers for returning students. Other district leaders, scrambling to prepare school buildings, map out bus routes and procure safety supplies, feared they were running out of time to safely reopen. Most don't want to follow the trend of schools and universities around the country that have reopened only to close a few days later amid an outbreak of the virus.

Scott Croonquist, executive director of the Association of Metropolitan School Districts, said school leaders are working furiously to sort out how many students and teachers will return, how many will stay home, and then match them up, like assembling a "giant jigsaw puzzle" as the clock ticks down to the start of the school year. The first day of school for most Minnesota districts is Sept. 8.

"Several districts came to the conclusion that they just needed more time, and are going to start the year with distance learning," he said.

Among the districts that recently changed plans: Bloomington Public Schools, where school board members voted Aug. 17 to start the year with students at home — two weeks after the district had said it would start in a hybrid format. Roseville Area Schools said in late July that it would bring students back part time for hybrid instruction — and then changed course to distance learning on Aug. 18.

Leaders of Mounds View Public Schools, who had also told parents to expect a hybrid format, announced that the district, too, would opt for distance learning. An Aug. 20 memo posted online pointed to concerns from families and teachers as a major factor.

"We want to take this time at the start of the school year to demonstrate that our plans meet or exceed the safety requirements from the Minnesota Department of Health and Minnesota Department of Education for the reopening of schools," it said.

Under Gov. Tim Walz's order for reopening schools, districts have some flexibility in their decision to fully or partly bring students back, depending on local spread of the virus and other factors. All districts must provide a distance-learning option for students, and sorting out how many students are coming back has taken up much of the month of August. At the same time, schools have been surveying teachers about whether they intend to return — and are only now figuring out that the math won't add up.

Complicating the equation: Especially in middle and high schools, teachers' licenses can be specific to subjects, which means if a chemistry or English teacher takes medical leave or requests to stay home, another teacher in a different subject can't easily be reassigned to fill the in-person teaching slot.

In Bloomington, about 30% of students plan to stay home, along with about 20% of teachers. District spokesman Rick Kaufman said school officials spent days trying to match up students and teachers, ensuring that they covered all the subjects and didn't have to use long-term substitutes — and couldn't make it work.

"At the end of the day we made the recommendation to the board that the alignment was just too difficult," he said.

In Roseville, about 40% of students intend to stay home, and more than 20% of teachers.

District spokesman Josh Collins said Roseville officials think they could sort out teacher assignments to cover all of the students at school, but felt it would take more time to ensure schools could safely reopen. Collins said the district wanted to avoid having to shut down soon after reopening.

"We want to make sure we offer some sense of stability for families," he said.

Bloomington and Roseville, along with other districts starting the year with distance learning, intend to revisit the decision later this fall and consider bringing students back.

Denise Specht, president of Education Minnesota, the state teachers union, said she's not surprised districts are doing an about-face on plans announced only weeks ago. She said many, hoping to give teachers and families enough time to plan, announced their reopening decisions before they'd gathered enough information.

In several districts, teachers have rallied to demonstrate their concerns about safety, calling for schools to remain online until the virus is more under control.

"My worry is the districts that are continuing to proceed forward without the staffing levels necessary" to open, Specht said. "What is day one going to look like for them?"

Meanwhile, schools in the southwest Minnesota communities of Ivanhoe and Minneota both announced late Sunday — the night before school was scheduled to begin — that they would delay the start of their school year to Sept. 1 due to COVID-19 infections.

"This is a reminder of just how fast things can change with this virus," said Minnesota Health Commissioner Jan Malcolm. "All of a sudden they need to turn on a dime."

In Facebook posts, Minneota Public Schools said a staff member tested positive, while leaders of Ivanhoe's Lincoln Elementary did not provide specifics about their case. Both posts said school facilities will be cleaned and sanitized.

Staff writer Glenn Howatt contributed to this report.