But if the drop between 2018 and 2022 was AS LARGE as the drop between 2022 and 2026, this implies that something happened that was as large as a global pandemic for students' education happened in the last four years. I believe the OP is implying this could have been AI. However, something else could be happening, such as use of phones and other devices or another fundamental problem in the education system.

Yeah, I am genuinely confused how the grandparent comment just glossed over it.

I believe that both COVID and LLMs are disrupting educational metrics. But in terms of which one did more short-term scale damage (i.e, within 4 years), I would wager it was COVID.

the event in 2020 would've only been the last straw in a series of bad policy in education. It might've brought forward this same outcome a few years in advance, but it isn't a root cause.