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いや、prep school はprivate or parochialだから、charterじゃないだろ。

charter school

noun　1.　an autonomous public school created by a contract between a sponsor, as a local school
district or corporation, and an organizer, as a group of teachers or a community group, often with a
curriculum or focus that is not traditional.

http://www.dictionary.com/browse/charter-school?s=t