Teaching martial art is more like being a "professor" than a "teacher". When professors encounter students they expect a willingness to engage in the material and a certain level of selfmotivation as the students invest in their learning. Teachers must expect to encounter resistance on a fundamental level and are expected by their superiors to teach everyone no matter what.
Martial arts teachers can eliminate dangerous or disrespectful students. Sometimes they can even get away with dishing out a serious beating. Teachers must eat it from the kid, then the defensive parent.
There are of ofcourse many great moments of learning in teaching anything. But teaching martial arts is gentle even with it's risks. That is of ofcourse unless you want to make a living.