One-fifth of American students, by our calculations, were enrolled in districts that remained remote for the majority of the 2020–21 school year. For these students, the effects were severe. Growth in student achievement slowed to the point that, even in low-poverty schools, students in fall 2021 had fallen well behind what pre-pandemic patterns would have predicted; in effect, students at low-poverty schools that stayed remote had lost the equivalent of 13 weeks of in-person instruction. At high-poverty schools that stayed remote, students lost the equivalent of 22 weeks. Racial gaps widened too: In the districts that stayed remote for most of last year, the outcome was as if Black and Hispanic students had lost four to five more weeks of instruction than white students had.

By our calculations, about 50 percent of students nationally returned in person in the fall and spent less than a month remote during the 2020–21 school year. In these districts where classrooms reopened relatively quickly, student-achievement gaps by race and socioeconomic status widened a bit in reading but, fortunately, not in math. And overall student achievement fell only modestly. The average student in the quicker-to-reopen districts lost the equivalent of about seven to 10 weeks of in-person instruction. (That losing just a quarter of a typical school year’s academic progress is a relatively good outcome only underscores the dimension of the overall problem.)

What happened in spring 2020 was like flipping off a switch on a vital piece of our social infrastructure. Where schools stayed closed longer, gaps widened; where schools reopened sooner, they didn’t. Schools truly are, as Horace Mann famously argued, the “balance wheel of the social machinery.”