A young person (not necessarily a schoolgirl) raised in a strict moral upbringing (not necessarily a Catholic) who acts completely opposite to the views they were taught in a way that is generally out of control and without care. Because they were taught that everything is equally sinful, they feel that any and all actions are morally equal (ie: kissing a guy is on par with sleeping with the football team) and not taught control, restraint, or moderation and instead taught only abstinence (ie: excessive uncontrolled drinking).
Betsy used to be such a nerd and a Bible Thumper. I heard she's doing lines of blow off the asses of prostitutes now. Man, she suffers from Catholic Schoolgirl Syndrome baaaad!