I mean if you're willing to accept that the best teachers are the ones that improve standardized tests the most then the research on VAM shows that good and bad

I mean if you're willing to accept that the best teachers are the ones that improve standardized tests the most then the research on VAM shows that good and bad teachers exist, it's just not able to reliably differentiate between them in a reasonable amount of time. A more straightforward 'proof' would just be looking at all students who take a class on something random like mycology and then seeing what percentage have had some level of engagement with that subject five or ten years later.