Do you actually believe what you just said? This is a sitcom, and in every sitcom kids move up a grade not stay in the same for 3 years in a row. In real life people ages and characters on tv have to age too because the actors who plays them also ages. Name me a show where a character stayed in the same grade for over 3 year period. And what about the other characters, like Fonzie, Chachi, Richie and the others, 3 years in real life is 1 year on tv for them too?