Seventeen out of 22 faith schools previously inspected by the now defunct Bridge School Inspectorate (BSI) have been rated inadequate or requires improvement by Ofsted.
Inspectors found some schools were failing to promote "fundamental British values", while one failed to ensure pupils were receptive to other people's beliefs.
Ofsted started inspecting the Christian and Muslim schools after the BSI lost its approved inspectorate status last September, over concerns around its inspection findings and inspectors.
Of the seven Christian and 15 Muslim schools inspected, one school was found to be outstanding, four were rated good, eight required improvement and nine were found inadequate.
Leadership was inadequate or requiring improvement in 17 schools, safeguarding arrangements were ineffective in eight schools, and there was inadequate practice in promoting fundamental British values in four schools.
In a letter to education secretary Nicky Morgan, Her Majesty's Chief Inspector Sir Michael Wilshaw said: “While the majority of the schools inspected were promoting fundamental British values, four schools were not. Seven hundred children attend schools where inspectors considered that pupils were not being adequately prepared for life in modern Britain. This is deeply worrying given our national focus on this work over the past year.”
The letter says that at one Islamic high school in Birmingham, a female governor sat out of sight of the male governors during a meeting with inspectors – with her contribution to the meeting made through a doorway.
It adds that a Christian primary school in Surrey has not ensured that pupils are “receptive to the views of people who may have different beliefs and values to their own”.
In the eight schools that failed to meet two or more standards for safeguarding, inadequacies included: an absence or haphazard recording of checks on suitability of staff to work with children, poor recording in pupils’ attendance records and ineffective training.
Ten schools were criticised for failing to provide a sufficiently broad curriculum, including four which did not enable pupils to develop aesthetic and creative skills.
The inspectorate estimates that more than 2,000 pupils were being educated in 17 schools where education was not good enough and of these, 1,019 were in the nine schools which were judged inadequate.
Sir Michael said his “serious concerns” meant he was recommending that re-inspections of schools previously inspected by BSI should be prioritised.