International Covenant on Econ, Soc. and Cult. Rights
http://esra-media.tripod.com/International_Covenant_of_Economic_Social_Cultural_Rights.pdf Art. 13(3) of this covenant states that the state must respect the liberty of parents ... to choose for their children schools other than those established by public authorities which conform to such minimum educational standards as may be laid down or approved by the State and to ensure the religious and moral education of their children in conformity with their own convictions.