Fast forward to the final two years of high school. I was much healthier, didn't miss school but did terribly. Maybe undiagnosed attention deficit, maybe depression, which led to a lack of motivation once my grades started to slip + general adolescence and socialising.
That being said, I had a relatively privileged upbringing and depending on your definition(s) of intelligence I would be generally considered above average. (I would hazard a guess that, a lot, if not most of HN leans that way) If either of those things didn't hold I probably wouldn't be where I am today.