Does anyone know the title of this eighties movie?

The basic plot was, there was a snooty rich girl and a nerdy not-rich girl who foguth allthe time in school...and i can't remember exactly why, but for some reason they had to go live at each other's houses for a while. in the end they learn about each other and become friends. The nerdy girl's name was Terri and i THINK the rich girls name was karen, but i might be wrong...terri had dark hair and "karen" was a redhead. it was from the eighties or maybe the late seventies, and i cant for the life of me remember what its called. Does it sound familiar??