Oh, yes. I had a friend who was kept back twice — just not so bright, according to the school. Actually, it was undiagnosed dyslexia. They never bothered to teach him algebra, but he looked over my shoulder when I was doing quadratic equation homework and he could solve the formulas in his head without ever having seen anything like that before!

This was in the early 1950’s; I assume at least dyslexia would be recognized today.