The Collaborative International Dictionary of English v.0.48:

Public school \Public school\
      (a) In Great Britain, any of various schools maintained by
          the community, wholly or partly under public control,
          or maintained largely by endowment and not carried on
          chiefly for profit; specif., and commonly, any of
          various select and usually expensive endowed schools
          which give a liberal modern education or prepare
          pupils for the universities. Eton, Harrow, Rugby, and
          Winchester are of this class.
      (b) In the United States, a free primary, grammar, or high
          school maintained by the local government.
          [Webster 1913 Suppl.] Public-service corporation

WordNet (r) 3.0 (2006):

public school
    n 1: a tuition free school in the United States supported by
         taxes and controlled by a school board
    2: private independent secondary school in Great Britain
       supported by endowment and tuition

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "public school":
   Gymnasium, Latin school, Realgymnasium, Realschule, academy,
   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,
   grammar school, high, high school, informal school,
   intermediate school, junior high, junior high school,
   middle school, night school, nongraded school,
   open classroom school, pensionat, platoon school, prep school,
   preparatory school, private school, progressive school,
   school without walls, secondary school, seminary, senior high,
   senior high school, special school, storefront school,
   street academy, summer school, union school, vacation school