Phrase used to describe girls who are younger than 18 (still in high school). Usually used among guys at restaurants when checking out girls.
Stems from Central Pennsylvania, specifically State College, home to State College High School. Girls that don't go to Penn State University, usually attend State College High School. Therefore, it is important to distinguish between the two.
Guy 1: What do you think of that chick over there, pretty hot huh?
Guy 2: Yeah she definitely hot, but do you think she's State College High?
Guy 1: Good call, she's a little young.