Least affected by nepotism within the institution, sure, but obviously affected by their relationship with their kids. Surely that would create a bias in favor of teachers who are likely to give students good grades?

If the education is subpar, the particular skills and knowledge that were missed can probably be picked up later as needed, but poor grades can alter the trajectory of your life in ways that are harder to recover from. So I'd expect parents who care more about the best possible outcome for their children more than being conscientious (which I honestly assume is a majority) to prefer those.