Covid Closed the Nation’s Schools. Cleaner Air Can Keep Them Open.

On a sunny afternoon in a cluttered music room at East High in Denver, two sophomores practiced violin while their music teacher, Keith Oxman, labored over a desk in an adjoining office.

The ceiling fans were off to prevent the sheet music from scattering. The windows were sealed shut. East High is Denver’s largest high school and among the oldest, and there is no modern ventilation system.

As the pandemic broke out, Mr. Oxman, 65 and a cancer survivor, feared getting sick or carrying the virus to his 101-year-old father. So he left the school when it first closed, in March 2020, and did not return for more than a year, staying home during later virus surges.

“We were supposed to have the windows open,” he said. “But the windows don’t open.”

Poorly ventilated spaces offer ideal transmission conditions for the coronavirus, and at the height of the pandemic, schools like East High were a searing point of controversy. An outbreak that began in November 2021 sickened more than 500 students — about one in five — and 65 staff members, one of whom died.

The pandemic led to…