Interestingly, the scores have been in free fall since the pandemic started and cannot be explained by learning loss from lockdowns as the effects are observed even after in school learning has resumed.

One possible explanation is cognitive loss due to covid infections:

https://www.health.harvard.edu/mind-and-mood/severe-covid-infection-may-lead-to-noticeable-cognitive-loss#:~:text=The%20COVID%20survivors%2C%20especially%20those,be%20about%2010%20IQ%20points