COLUMBUS, Ohio — The infection of a single cafeteria worker was all it took to close classrooms in the small Lowellville school district in northeastern Ohio, forcing at least two weeks of remote learning.

Not only did the worker who tested positive for the coronavirus need to quarantine, but so did the entire cafeteria staff and most of the transportation crew, because some employees work on both. The district of about 500 students sharing one building had resumed in-person instruction with masks and social distancing and avoided any student infections. But without enough substitute workers, administrators had no choice but to temporarily abandon classroom operations and meal services.

“It boils down to the staff,” Lowellville Superintendent Geno Thomas said. “If you can’t staff a school, you have to bring it to remote.”

Around the country, contact tracing and isolation protocols are sidelining school employees and closing school buildings. The staffing challenges force students out of classrooms, even in districts where officials say the health risks of in-person learning are manageable. And the absences add to the strain from a wave of early retirements and leaves taken by employees worried about health risks.

It’s another layer of the “tremendous stress” faced by administrators and educators navigating the pandemic, said Dan Domenech, executive director of AASA, the nation’s leading school superintendents association.

The superintendent in Groton, Connecticut, recently announced the entire district would transition to distance learning for two weeks following Thanksgiving — a decision driven primarily by a staffing shortage.