Inspectors found that the children "are not taught explicitly about issues such as sexual orientation.
“This restricts pupils’ spiritual, moral, social and cultural development and does not promote equality of opportunity in ways that take account of differing lifestyles."
The inspection was carried out at the request of the Department for Education to check the school’s progress towards meeting independent school standards.
Ofsted said the school acknowledged that it did not teach about “the protected characteristics as set out in the Equality Act 2010, particularly those relating to gender re-assignment and sexual orientation.
"This means that pupils have a limited understanding of the different lifestyles and partnerships that individuals may choose in present-day society."
An Ofsted spokesperson said: “Children living in England deserve the best - the law expects schools to demonstrate that they are encouraging pupils to take a respectful and tolerant stance towards those who hold values different from their own.
“Ofsted acts robustly and impartially to ensure children in England receive a good education.”
Inspectors did praise the school for teachers’ "good subject knowledge” and its “high-quality classroom resources".
Inspectors also noted that since the last inspection the school had improved in areas such as safeguarding and leadership.
Private schools that do not meet Ofsted's standards must show signs of improvement or face closure by the DfE.
The school declined to comment.
