Hi,
Not sure I can add to the topic, but the reverse is also true, since the first guitar I ever had was a Gibson EB-0 (with the pegs the other way!), and I had it for over 5 years, and my only issue, really, was no lessons, and little ability to take it further, what with work full time and school full time, and all that. In the end, what I did not know, was, what a great instrument it was, although I was well aware of how many bands and folks I was aware of, were using that same bass guitar and sounding great, which I could not achieve and did not know enough to get that far.
So I traded it in for a Fender Jazz ... and while it was a great bass and I had it for 20 years, the FJ did not help me improve, because I could not "hear" it, or "feel" it, although with a couple of teachers I obviously had a touch, that was nice ... but what I wanted to hear and play, was not available, and no one was able to help define that better, so I could improve my interest, and then (hopefully) develop it.
I think that most of it, has to do with your own inner vision, and feel. If you notice the kid just fooling around in the background, and finding things, instead of that extra time with one more scale, I would suggest that you want to see if the kid is "looking" for something in that sound, which could/should help improve the interest in the instrument and its sound. My main concern, is that most teachers do not see enough of those moments, and are not capable of helping the youngsters learn from it, and develop it into something ... different ... that helps the kid maintain the interest. I, simply, was not interested in learning when the saints go stupidly nowhere or sweet little eighteen (adults only!) ... and to me, that turned me off any more lessons, while I tried hard to learn the bass parts in many a Guru Guru song (did not exactly succeed ... the variations are way out of sight and differ all over the place), or play along with Fairport Convention or even some Pink Floyd ... but a teacher, instead of helping me clean up those to make it better, so I would learn more on the instrument, would take me out of that space, and bring in some easy 4/4 thing or other that took away what I wanted to work on.
It did not dawn on me, that I had to have some mechanics down, mostly because I go for feel so much, and I could go a couple of songs on Nektar, but a Chuck Berry ... I simply can not get inspired to even learn the notes!
Sometimes, I wonder what kids are looking for ... a sound that is on their head? ... if that is so, what you are trying to teach is not gonna happen ... the kid will still be tied up to what he is hearing, even if it is his favorite song. Help him with it, then?
Teaching changes then ... you are no longer a teacher, but an assistant helping the kid learn more ... and simply thinking that finger mechanics is the answer, is a harsh reality that takes the dream, the vision, and the inspiration out of a lot of music! Without that 3rd dimension, the music dies!
Not sure this helps ... may sound a bit winded, but it's not meant to be. And the same thing is happening with me trying to get some help on the keyboards/computers!