I did, however, have a teacher who taught an advanced subject and I found his instruction so good that I did not have to bother with homework and assignments if I was happy with B grades — as I wasn't particularly motivated, only occassionaly did I put in the effort for an A.

I could, however, see the level of preparation that he put into it. When students confronted him with a difficult task, he'd not attack it right away but instead prepare for it for the next class so he'd provide the most effective instruction (it was not about being embarrased to show how exploration is sometimes messy because he'd quote that as the reason he won't do it right away). He was also so focused that he kicked out a school director when he tried to interrupt class with some sales pitch for whatever.

Not everybody could score a B grade just out of his instruction, but nobody was failing a class because the instruction was so good.

I will also openly admit: I had exactly one instructor like this in my life, so it is a high bar to clear ;)