Something interesting has revealed itself to me as I've gained more experience as a teacher: the higher level you teach, the less it becomes necessary to be good at teaching.
To put it another way, when one teaches more basic material and/or younger students, the greater the focus will be on your TEACHING chops rather than your knowledge of the subject matter.
For instance, teaching children requires extremely high levels of organization, empathy, and need to create a plan for consistent improvement. Teaching adult beginners requires the knowledge that in fact they are professionals in their normal lives and are not accustomed to being a beginner at something. Teaching a college general education course to a class consisting largely of ESL students, first year college students, and students who are first in their family to attend college requires so much more beyond just knowing your subject material.
On the other end of the spectrum, when I've taken lessons with internationally celebrated jazz artists, it didn't really matter to me if they were expert teachers per se - I just wanted to spend an hour or two with them to get a glimpse of the elite musician's mind and to hopefully unearth some wisdom that would change my playing. As a professional artist myself, I don't need to rely on the teacher to provide structure, or to have empathy for my situation, etc. I just want the raw information so I can wrestle with it myself for the next couple of years.
When I've taught higher level musicians myself, I also assume that I don't need to do a lot of hand holding and that they're here for similar reasons as mine for studying with an artist who is further along than they are.
As I began teaching in community colleges I learned that the things that make the biggest difference in student's lives have to do with teaching "stick-to-it-ive-ness" and responsibility, while also being sympathetic to their needs and personal situations. Many of them are working through school and/or raising kids and/or coming back to college later in life and/or getting a second chance at college. The skills needed to address these things have very little to do with the actual subject matter one teaches. And this is in college.
Of course, if you're a Harvard professor that gets quoted in the news or is otherwise considered an expert then you're expected to have an unparalleled level of knowledge in your field - and then you will have a prestigious, world-class teaching post even if you may not be a world-class teacher.
Similarly, we all know that there are great players who can't teach their way out of a paper bag, but they're still busy teaching lessons because . . . they're great players, and attract students because of their public profile.
Perhaps the best teachers of them all are teaching K-12. That they are able to essentially wrangle cats and grow their minds is an enormous feat. Hats off to these tremendous superheroes. I am in awe of you.
What's cool is that there is so much you can accomplish as an artist-teacher. There are different avenues for changing lives and making music be a positive force on the bandstand and in the classroom. One of my teachers was Dr. Aaron Lington at San Jose State, who is a world-class artist-teacher and Grammy winner with Pacific Mambo Orchestra. My friend and member of my nonet Ben Torres also won a Grammy for his PMO efforts and is a great elementary school music teacher. Top that!
At the end of the day, all of us that aspire to be great artist-teachers need to understand that there is so much more to teaching than "knowing your stuff". It's a given that all professional educators have a baseline of knowledge, but it takes a while longer to figure out the other skills needed to become a truly great teacher.