As someone who has instructed high school kids for 30 years, I can tell you that there are kids who are gifted with coordination and ability to learn things quickly, and kids who need hours and hours of repetition to achieve one small thing, and kids who never seem to learn anything. On any giver multi-day canoe trip, I would have all three kinds.
These were the basics I taught before every trip to the newbies, in order of instruction: (lots of stuff was covered before this, like safety, camp procedures, etc, this was just for getting on the water) canoe parts, , names for parts of paddles, canoe carrying, canoe entry. J-stroke. After two kids out of 20 got the J stroke, I taught the stern pry to the rest of them. Probably 17 out of 20 would get the stern pry within a half hour. The other three became bow paddlers if possible. Then we did draws and cross bow draws, no pries. Then spinning the canoe 360, obstacle avoidance under full steam, side slips and canoe over canoe rescue. That was enough to get us going on a trip. After about ten days, most kids would be doing very well, but the goon-strokers were still in the majority. Most of them didn't pick up the J stroke until about grade 11, and many of them never got it.
As for the Canadian practice of prying off the gunwales.....well, I'm Canadian, and I do it. Why not make full advantage of a fulcrum? Seems like a no brainer to me. As for wearing out gunwales and paddles, I would have to disagree. I haven't worn out a single gunwale or paddle due to this in many 1000's of kilometers of paddling. I pulled out one of my old paddles I had made about 20 years ago, when I was paddling between 1000 and 2000 k a summer.
As you can see, the oil is worn off the shaft, but the shaft is still in very good shape. A re-oiling will bring it back to normal. I have broken a few paddle shafts leveraging off the gunwale, but only softwood shafts, never hardwood.
So to sum up my ramblings, for some reason, the J Stroke is a difficult thing for many people to learn. The Stern pry is not. If you can get someone going straight with a stern pry, wait for a few days or even weeks and then re-try the J Stroke.