Getting a measure of academic performance that applies to all kids across the United States is tough. But early in the pandemic, students’ test scores hinted at the academic blows that were to come.

Comparing previous test score changes of kids in California to the changes from fall of 2019 to the winter of 2020 showed an overall academic lag. “On average, kids are two to two-and-a-half months behind where we would expect they would have been if COVID hadn’t happened,” says learning science expert Libby Pier of Education Analytics, a nonprofit based in Madison, Wis. Considering that a normal school year is nine months long, “that’s a quarter to a third of the school year they missed out on.”

The pandemic, of course, didn’t end in 2020; measures of academic slipping got worse as time went on. Elementary school students across the United States finished the 2020–2021 school year an average of five months behind in math and four months behind in reading, an analysis from the analytics firm McKinsey & Company, headquartered in New York City, shows. Those numbers, described in a July 27 report, don’t reflect all students. The analysis counted results only from kids who were in schools to take the tests; kids at home might have fared worse.