Independent faith schools must ensure all resources used to teach about other religions are accurate, inspectors say.
Ofsted surveyed 51 schools in England from the Christian, Jewish, Muslim, Buddhist and Hindu faiths, at the request of Schools Secretary Ed Balls.
Inspectors said some materials were biased or had incorrect information about the beliefs of other religions.
But the schools were judged to be good at helping pupils understand the importance of being a good citizen.