We don't have any NAEP long-term test results for years between 2012 and 2020, because they were canceled due to budget cuts[1], so we can't use the linked data to determine whether the decline started before or during COVID.

We do have the NAEP main series test results[2]. At a first glance at the math results[3][4], it appears they peaked in 2013, then fluctuated through 2019, then dropped significantly in 2022 and somewhat rebounded in 2024, which really does suggest COVID.

[1]https://www.edweek.org/teaching-learning/debate-flares-anew-...

[2]https://www.nationsreportcard.gov/report_archive.aspx

[3]https://www.nationsreportcard.gov/highlights/mathematics/202...

[4]https://www.nationsreportcard.gov/reports/mathematics/2024/g...