The consulting firm examined spring 2021 test results for 1.6 million students in grades 1 through 6 across the U.S., then compared their performance with that of similar students pre-pandemic. They discovered that the pandemic-era children were, on average, about four months behind in reading and five months behind in math.

However bleak, these numbers “likely represent an optimistic scenario,” McKinsey says. The results measure “outcomes for students who took interim assessments in the spring in a school building—and thus exclude students who remained remote throughout the entire school year, and who may have experienced the most disruption to their schooling.”

The McKinsey study doesn’t say it, but teachers unions were the main architects of this calamity by first refusing to return to the classroom, then insisting on watered-down schedules. The data company Burbio found that, by the end of the spring semester, most students could attend school at least part-time. But due to union demands, the return sometimes amounted to a few days or hours of in-person learning a week.