I also started underachieving in 6th grade (age 11). I got mostly Ds and Fs in grades 7-9 and then changed schools and did somewhat better because I was happier (for nonacademic reasons). But I still would get Ds in classes I didn't like. This continued until I was in my third year of college when I switched majors to something I found quite challenging (biochemistry) and decided I wanted to apply to MD-PhD programs. I managed to get As in all of my biology and biochemistry courses, though my extremely weak math background made it impossible for me to do well in any math-based courses.
I think the reason for my underachievement was a mixture of extreme boredom, executive functioning issues, and a touch of unremediated dyslexia.
