all of the other definitions are wrong. a prep is someone who goes to prep school (andover, exeter, groton, nobles, deerfield, choate, etc) They do not wear trashy clothing like american eagle or abercrombie...they wear polo, j. crew, vineyard vines, sperry topsiders, chinos, oxfords, etc. preppy girls are not overly bitchy because to gossip wouldn't be classy. They maintain a bland facade, and are more often vapid than the cheerleader princess.
Although Ashley provoked her and started yelling, Louisa, like a true"preppy girl", walked away from the scene and mentally noted to not invite her to go sailing on Saturday at her Cape house.