Schools shuttered in rural Dorchester County on the Eastern Shore of Maryland this week amid a rise in coronavirus cases, more than a month after students began a gradual return to in-person classes.

The county is the first to retreat from classroom learning in Maryland since state officials began urging school systems in late August to reopen school buildings for the fall.

Dorchester County School Superintendent W. David Bromwell said about 20 percent of the school system’s 4,700 students were on campus part time — a number slated to more than double next week under a hybrid-learning plan for pre-K to grade 12.

“We were doing well and moving slowly,” he said. But a sudden spike in the prevalence of the coronavirus brought the plan to a halt.

As of Friday, Dorchester led the state for new cases per 100,000 population during the last seven days, at 251, according to data compiled by The Washington Post.

The test-positivity rate jumped from 2.9 percent to 6.1 percent in a eight-day period, said Dorchester County Health Officer Roger Harrell.

Coronavirus cases have not spread in schools, school and health officials said. They called the closure a cautionary measure taken to prevent a school outbreak.

“The scary part is how quickly it flipped, and it seemed to be growing exponentially,” Bromwell said. “It just appears that it’s hitting rural America.”

The sharp increase has not been traced to a particular event or outbreak in any part of the county and has affected people across age levels, Harrell said. The county includes the city of Cambridge, amid an expanse of farmland and waterways.

“We’ve not really figured out why,” he said. “I wish we had the magic answer, but we don’t have it yet.”

It will take at least two weeks of consistently lower positivity rates before schools can reopen, officials said.

At that point, Thanksgiving — and the possibility of spread during family get-togethers — may be around the corner and “certainly a concern,” Bromwell said.

“It kind of takes the wind out of your sails,” he said. “You start to get the impression that you’re returning to normalcy, and then . . . it takes the wind out of you.”

In recent months, Gov. Larry Hogan (R) and State Superintendent Karen Salmon have visited schools around the state, lauding efforts to revive classroom instruction. Hogan said in late August that school systems were fully authorized to begin safely reopening for in-person classes, based on improving health metrics.

Nineteen of the state’s 24 school systems have opened school buildings to students to some extent this fall, state officials said Friday.

Hogan’s office issued a statement Friday saying Dorchester’s approach is consistent with data-driven health metrics provided by the state.

“The recent rise in the county’s positivity rate is connected to a small number of family clusters, which is in line with trends we are seeing statewide,” spokesman Mike Ricci said.

Salmon called the changes in Dorchester “an example of the metrics being utilized to inform health-based decisions at the local level,” according to a statement provided by the Maryland Department of Health.

Dorchester opened Sept. 8 and soon brought back seniors in career programs and later students with special needs. More recently, it embarked on a hybrid approach that combined online and in-person learning for students in pre-K, kindergarten, sixth grade and ninth grade.

Since schools opened, nine people related to schools in Dorchester have tested positive: four students, all teenagers, and five employees, only some of whom worked in school buildings.

The announcement about closing school buildings was made Wednesday night following a decision by the superintendent and county health officer. Students had remote learning Thursday and Friday, and will be in all-virtual learning in the days ahead.

Katie Holbrook, president of Dorchester Educators, the county’s union for teachers and support staff, said many were disappointed that classroom learning was being postponed but also glad the safety of students and staff was prioritized.

“It’s being proactive instead of waiting for a large outbreak in the schools to take action,” she said.

Linda Barnes, a parent and educator, said she appreciated the difficulty of Bromwell’s decision, which was causing some buzz on social media but was best for health and safety.

Still, she said, “it’s sad” thinking of her seventh-grade daughter. “She was really looking forward to going back, and she really missed her teachers and her classmates.”

Meagan Fitzpatrick, an infectious-disease transmission modeler at University of Maryland School of Medicine, said school and health leaders acted wisely in changing course when the numbers shifted. “When we open school districts, we should have protocols to close like this,” she said.

Dorchester is not the only county school system to face changes in its in-person learning plans because of the coronavirus.

Elsewhere on the Eastern Shore, Caroline County’s Greensboro Elementary School, which had 400 children on campus, was shut down Oct. 19 for two weeks after eight staff members tested positive for the coronavirus in one week.

The school was at the center of a Post story about bringing small groups of children with high needs back into school buildings.

Since Caroline County schools opened in early September, 14 staff members and one student have tested positive, Sandi Barry, a spokeswoman for the school district, said Friday.

State education officials said they commend Greensboro Elementary “for tracking, reporting, and communicating covid infections within its school community and respect its decision to close temporarily to in-person learning.”

“In this new education environment marred by the covid virus,” they said in a statement, “we must remain vigilant and flexible and make responsive local decisions that are in the best interest of students, teachers and staff. ”