I disagree with the grandparent comment, but you could imagine a few teacher related causes:

- Maybe the better teachers retired or quit due to burnout/better opportunities (the pando surely increased both of these and I think one could imagine reasons for better teachers to disproportionately stop teaching). One thing we observe is the best decile being a lot less affected than the average or worst, which vaguely lines up with the hypothesis. I didn’t try to look into attrition or retirement rates.

- There could be a new teaching method that is popular with teachers, teachers’ unions, administrators, or governments but which is actually quite ineffective. My weak understanding is that the quality of education research is very poor and that evidence-based decision making for how to teach is extremely rare, so this also feels plausible. I think many HN readers would have seen stories about math teaching in SF public schools, and there’s also the positive case of Mississippi seeing great successes from some teaching methods that are supported by good evidence. In some sense this isn’t really a case of teachers being bad.