I wonder if the increase in ADHD diagnoses can be tied to school becoming more focused on sitting still in a room for a long period of time. At least for me, I was doing really well in school when classes were ~50 minutes, and some included phsical activity like shop class and home economics. In my junior year of high school, they switched classes to 90 minutes, you took different classes on different days, and none of my classes included physical activities. My grades plummeted after the switch, and I ended up getting diagnosed with ADD when I was struggling with college in my mid-20s.