Here's one of those challenges every teacher faces:
When to "Tunnel In" and when to "Fly Over."
"Tunneling In" happens when I, as a teacher, spot some fundamental thing that needs work. It might be a rhythmic problem or a breathing problem or an articulation problem. Whatever it is, I know that I'll need to step away from the solo/etude/excerpt and address this issue. While this kind of work is really valuable, it also means that we may get bogged down and never progress through much music.
One of my colleagues (different instrument, different school) once told me that he felt one of his secrets was making the students constantly prepare new material. I think he was going for three new things every week.
Just as a practical matter of having a certain volume of material to prepare each week means that, in lessons, the teacher won't be able to Tunnel In.
"Flying Over," of course is the opposite. It means getting coverage on a larger amount of music. It means the teacher will have to make cursory comments about general sorts of things.
To be a good teacher, I think you must do some of both. As you work with a student, you need a sort of mental pie chart of your lesson time with that student. How much time have you spent in each activity?
A Couple Somewhat-Related Thoughts:
One of my frustrations with master classes at conventions is that, too often, the teacher never tunnels in. Maybe it's too difficult with an audience looking on. Maybe they're worried that the problem is too large and thorny to be handled in that setting. Instead, one usually hears cursory sorts of comments that often reveal very little. Sometimes I'd like to see how an advanced teacher tunnels in.
Years ago, I was teaching a younger student who had some terrible tension issues. My usual bag of tricks just didn't seem to be working. I arranged for that student to have a lesson with a master teacher (David Fedderly) while I observed. It was a wonderful experience to watch a more advanced teacher address problems I had struggled to fix. I would recommend this for others as well. It does involve checking your own ego at the door but it's worth it.
When I first started teaching full-time at the college level, I found many of my students had very weak fundamentals. I went into major "tunnel in" mode, spending large chunks of lesson time on easier material so they could develop a more relaxed, efficient approach. Guess what happened? Some were helped but too many others reached the conclusion that, since my stuff was "easy," they didn't need to spend much time on it. Their jazz band charts, on the other hand, were very difficult so that's what they practiced. Did those basic problems get fixed...for many of them: not really.
Psychologist Mihaly Csikszentmihalyi (“cheeks-sent-me-high"), author of the book
Flow, had an interesting chart that shows that balance between boredom and anxiety. It's not exactly what I'm talking about but it's great food for thought...