It seems to me that calculus is placed on a pedestal as something unachievable by the masses. But looking at it, I see a lot of material in the calculus that could be approached by students on a sixth or seventh grade level. If they can graph a straight line and a parabola, they are game for calculus, at least some calculus.
The second point is teaching to the test versus enrichment. The question may be how can one jump start or help to kindle a person's intellectual curiosity. This will depend partly on the students interests, partly on his innate intelligence, and partly on the assets of the teacher/instructor/tutor, and the assets of the school (computers, laboratory equipment, etc.).
Each case is different, and each teacher is different. There is a lot of trial and error involved. I doubt very much that, in spite of all the "research" being done, one will ever systematize a method for teaching that is 100 percent effective and efficient. There are too many variables involved.