Where did all these teens come from? Harmony Colangelo, co-host of This Ends at Prom, is here to explain how, before Americans got to worry about what teenagers were up to, we first had to decide what they were—and how a boom in postwar educational films taught a generation of adolescents what not to do.Skipper Learns a Lesson: https://www.youtube.com/watch?v=h8kJzBJrOkU1950 Family Date…