I am in the education industry, I have instructed and I am an instructional designer. Not that this makes me authoritative in any manner, but, everyday, I do live with the perspective where some students are better than their instructors.
When it comes to education I usually tend to use the example of a Basketball coach. Even in High School, most players on the team can beat the coach on a one on one.
However, I feel that instruction is different. As the question asks what is the qualifications for a good "teacher." In this case I will yield to my example above. Just as a music teacher in most settings above high school cannot outplay most of their students.
Now for the down beat. What we call "teachers" should probably be referred to as "INSTRUCTORS." Instructors do their fair amount of teaching, however, teachers usually do not do that much instruction.
If I wanted to find someone to help me play the drums better, I would seek out an instructor. Someone who has the ability to "show me"
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The next step.
Some of us will improve to the point where it is going to be difficult to find someone who has better chops than you. Who does someone like Steve Smith turn to.
Now I will use the example of a Golf Pro. A good golf pro can look at someone like Tiger Woods, (who he will never be able to match on the golf course,) yet still help him improve his game.
There are drum instructors out there that can do the same..
I know a drum instructor, for instance, that one drum lesson from him and you will take three strokes off your game. @:-O -- You get my point though!?