Preppies are people who go to prepatory highschools (Groton, Hotchkiss, Philips Exeter, Philips Andover, Deerfield, Laurence, etc.). Most of them shop at J. Crew, Banana Republic, Brooks Brothers, L.L. Bean, Ralph Lauren and all designer stores. Preppies are often mistaken for kids who shop at Abercrombie & Fitch, American Eagle Outfiters, Rave, etc. Those who think "preppies" are girls who shop at those stores are VERY wrong.
You could tell by the way they were clad in Brooks Brothers that they were utter preppies.