Thanks for writing this. This puts some of my experience in perspective. When I was in 11th grade, I was doing very poorly in math: I was barely scraping through the exams and my math teacher told my parents that I would not be cut out for college in engineering or physical sciences. But come 12th grade, I was on top of the class without even breaking a sweat; even though the math got much harder and the math teacher was the same. Now, I'm doing a PhD in physics.

I think an important factor in my case was finding friends who were also genuinely curious about math, instead of just wanting to get through the exams.

But I still think there were a lot of hidden variables that governed this transition that I'm still unaware of. For example, friends cannot explain all of it as I had many of the same friends in 12th grade as well. "Increased motivation" is not really an explanation. Learning more deeply---from different sources and in interesting contexts--- are significantly causally linked with more motivation.