Is educational achievement closely correlated with some teachers and not others? Is the correlation confounded by other variables like the student’s family income or choice of breakfast cereal?
Most of the political discourse tends to treat all of those situations as the same.
(Growing up, my classmates were mostly in the "sufficient positive influences" camp, and yeah, the teachers who had big impacts tended to be the really bad ones who destroyed momentum/interest in a topic for one or more of us. There were also high-impact inspirational teachers - but they were more of an "alter the trajectory of what someone wants to do in their life" sort of thing, which isn't really measured by standardized tests.)
There are bad teachers. I've encountered a few that few students could learn from. Isolating this effect from the effect of the outside factors would be very hard, though.