I don't know. The worst teacher I ever had, from what I recall, had a fundamental problem of being unable to control the children in his class. It didn't help that, IIRC, he was fairly softly spoken. Discipline, to put it frankly, didn't come naturally to him. For all that I wasn't the best pupil in the world, I did actually want to learn, and his classes seemed to just devolve into utter chaos, in which environment it was difficult to hear what he was saying.
Whether I'd tell him that if I had a chance, I don't know. As it is, I mentioned him on Friends Reunited a few years ago and was told that he'd died.