Green’s Dictionary of Slang

(US) a generic for a wholesome, middle-class sorority girl; also as adj.

1931		Michigan Alumnus XXXVII 277/3: In a late bulletin he opines that the boys at Harvard, Yale and Princeton might be better off if Betty Coed didn't bother about having crimson lips.
1938		‘Campus Lingo’ in Reading (PA) Eagle 20 Mar. 7/2: Strange indeed is the campus lingo these days of Joe College and Betty Co-ed.
1947		film title Betty Co-Ed.
1961		F. Kohner Gidget Goes Hawaiian 20: You look so – so – Betty Coed.
1970		Current Sl. V:3 5: Betty-co-ed, n. A college girl who overdresses; a girl who is dressed up even for everyday occasions.
1974		D. Mamet Sexual Perversity in Chicago (1994) 50: Betty Coed from College, regular young broad.