When the bell rings at 2:25 p.m., math teacher Melissa Brown watches from a window as kids file out of Riverton High.

“The first thing they do,” she said, “is rip off their masks. The second thing is they hop in a car with a bunch of their friends like COVID doesn’t exist once class is over.”

Her school is one of the 53 high schools, or roughly a third of those in the state, that have crossed the 15-case coronavirus threshold that state officials say should trigger two weeks of online instruction.