Teaching is a tough job. Managing a classroom with the types of behavioral and attitudinal patterns that are common these days is no easy feat. And doing all that while actually teaching something is even harder. Now, who should be teaching such classes. The relative of the Menahel/Rosh Yeshiva, or someone that has been trained in the field of teaching? Now you know……the rest of the story.
A good teacher can manage the difficult and challenging student. More often than not, this doesn’t happen, since most teachers lack training. Additionally, the teacher who needs to reach the tough student may need to expend a little energy and time to make the connection outside of the classroom. Who wants to do that? The teacher who just needs a job because they exhausted their time in kollel, or someone who is dedicated to the mission of educating young people regardless of their degree of challenge?
The answers to the questions posed here are also solid replies to the OP.