My parents couldn't figure out for the life of them why I was suddenly struggling and thought I was having adjustment issues. I had taught myself to read when I was 3; how could I suddenly be having trouble keeping up?
It took longer to figure out because I was only nearsighted in one eye. I was tall for my grade, so as long as the person in front of me to the left was shorter than me or the teacher was writing high enough on the board, I was fine, because my left eye was fine. But when everything aligned just wrong, I was suddenly helpless, because my right eye could barely see clearly an arm's length from my face! It's a hard thing to notice when only one of your eyes isn't working very well, especially when you're 9.