WordNet (r) 3.0 (2006):

summer school
    n 1: an academic session during the summer; usually for remedial
         or supplementary study

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "summer school":
   alternate, alternative school, blab school, boarding school,
   central school, consolidated school, continuation school,
   correspondence school, country day school, dame school, day school,
   evening school, extension, finishing school, free school,
   informal school, night school, nongraded school,
   open classroom school, pensionat, platoon school, private school,
   progressive school, public school, school without walls,
   special school, storefront school, street academy, union school,
   vacation school