The report shows that some kids who were already low performing lost even more ground than students at the top of their class. This was especially true in math for fourth graders.

“Particularly in math, there are a set of foundational skills that build on each other. Those students in particular didn't have as much of an opportunity to catch up with some of that unfinished learning,” said Chase Nordengren, PhD, the principal research lead for Effective Instructional Strategies at NWEA (a nonprofit that creates academic assessments for students pre-K-12).