All four characters were phased out when the show was re-tooled starting with Season 2 (when Jo made her debut). Molly's final appearance was in Episode 2.2 ("The New Girl: Part 2").
Nancy, Sue Ann, and Cindy made occasional appearances through Season 3, and all three returned in Episode 8.6 ("The Little Chill"). Molly was mentioned in that episode, but did not appear (aside from flashback scenes, possibly due to actress Molly Ringwald's film schedule).