Cincinnati Kids - Marcia and Jan, Jan, Jan!
https://www.imdb.com/title/tt0531129/mediaviewer/rm3844992513/
Proof of two things:
1) Jan did "grow up" faster than Marcia in the final season.
2) No way Marcia is wearing a bra, or else it's a very cold day at King's Island! [Which it was not]
[Happiness is seeing that IMDB has individual show entries!]