WordNet (r) 3.0 (2006):

preparatory school
    n 1: a private secondary school [syn: preparatory school,
         prep school]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "preparatory school":
   Gymnasium, Latin school, Realgymnasium, Realschule, academy,
   grammar school, high, high school, intermediate school,
   junior high, junior high school, middle school, prep school,
   public school, secondary school, seminary, senior high,
   senior high school