WordNet (r) 3.0 (2006):

middle school
    n 1: a school for students intermediate between elementary
         school and college; usually grades 9 to 12 [syn: secondary
         school, lyceum, lycee, Gymnasium, middle school]

Moby Thesaurus II by Grady Ward, 1.0:

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