Ho trovato questi riferimenti in rete:
http://en.wikipedia.org/wiki/Charter_school
http://archivio.invalsi.it/chi_siamo/eventi/conv_mag_2004/do...

Buon lavoro!

Secondo me non è proprio traducibile! Io lascerei in inglese... anche perché segue la spiegazione di cosa si tratta, quindi meglio di così...
Ciao, buon lavoro
Luca

http://www.scuolaer.it/page.asp?IDCategoria=131&IDSezione=38...
Charter schools are nonsectarian public schools of choice that operate with freedom from many of the regulations that apply to traditional public schools. The "charter" establishing each such school is a performance contract detailing the school's mission, program, goals, students served, methods of assessment, and ways to measure success. The length of time for which charters are granted varies, but most are granted for 3-5 years. At the end of the term, the entity granting the charter may renew the school's contract. Charter schools are accountable to their sponsor-- usually a state or local school board-- to produce positive academic results and adhere to the charter contract. The basic concept of charter schools is that they exercise increased autonomy in return for this accountability. They are accountable for both academic results and fiscal practices to several groups: the sponsor that grants them, the parents who choose them, and the public that funds them.

così si coglie l'informazione generale ma caratterizzante del tipo di istituzione. meglio , se possibile, fornire sempre una traduzione!