Why'd they get rid of Richie?

Just speculation on my part, but I wonder if it had something to do with Howard Rice aging visibly during his time on the show. I always thought of Richie as being a year or two younger than the other kids, but by the end of the second season, he looked as old as the oldest kids. Maybe the producers wanted to bring in another actor to restore the dynamic of having a younger-seeming student interacting with older kids. Or maybe Howard Rice decided to move on.