Even put them in junior high school. Make lisa 6th grade, Bart 8th grade. idk what people think Junior high schoolers do, but it's pretty much skate, hw, video games, get into trouble watch television ect. The same stuff they already have the Bart and Lisa do.
The weird part is that junior high school strikes that perfect balance between not being TOO old and forced to abandon more childish stuff the writers can incorporate while at the same time able to have romance drama, more adult activities, part time jobs, more adult situations ect.
The same applies to South Park imo. Why not just make the kids 6th graders?