Albert Lea Area Schools had 36 cases of COVID-19 and 290 students in quarantine just five days after classes started on Aug. 19, far outpacing infections at the start of the last school year.
The district did not have a mask requirement, but one will start on Monday in grades six-12, where most of the cases originated.
"I just had no idea that it was going to take off like it did so fast," Superintendent Mike Funk said. "That was a surprise."
The district is in Freeborn County, which has one of the highest COVID-19 transmission rates in the state and where 57% of vaccine-eligible residents have completed the vaccine series, compared with 66% statewide.
But state officials are concerned about more school outbreaks after most districts start classes after Labor Day, especially in districts that will not require masks in high transmission areas, which includes most areas of the state.
"We've heard some particularly troubling news from Albert Lea," Minnesota Health Commissioner Jan Malcolm said. "The transmission chain started just that quickly and now is not easily stopped."
Funk said many districts in the region have mask recommendations but not mandates, reflecting community opposition to them.
"In outstate Minnesota it is a different world than it is in the metro in terms of beliefs about mask mandates," he said.