This is a great question.
Can't really good instructors teach really good skiers as much as they want if they get in the circuit that does professional clinics or if they are willing to coach racers? And my sense is that most racing programs are constantly trolling (good trolling, not bad trolling) for good coaches. I don't know how hard it is to find clinicians for PSIA events, etc.
Or are you interested in how many of the students who take lessons through a ski school are either good or really good?
And, not wanting to open a whole can of worms, good and really good are probably better catagories than beginner, intermediate, advanced, expert, etc. Although I've rarely heard anybody over the age of about 15 admit that he or she is "good, but not really good."
Finally, and this is my own twist on your question, what's the difference between a really good student and a really good skier?