The Guru Nanak school in Hillingdon, west London, is effectively only open to practising Sikhs as its admissions policy gives priority to committed believers.
The school, which has only 450 places, is popular with parents and achieved the second-best value-added GCSE results in the country last year. But teachers admit that its high standing with Sikhs gives other religions little or no chance to join the school, harming its ability to expose pupils to other cultures.
Now the school is writing to the local schools organisation committee, which oversees admissions, asking to increase the number of pupils from other faiths.
Earlier this year David Bell, the chief inspector, caused a furore by saying that private Muslim schools are threatening social cohesion by failing to teach about other cultures.
Ofsted's figures showed that 36 per cent of Muslim schools were failing in their duty to "assist pupils to acquire an appreciation of and respect for their own and other cultures, in a way that promotes tolerance and harmony". Evangelical Christian schools fared even worse, with 42.5 per cent failing to meet the criterion.
Greg Hall, deputy head of Guru Nanak school, said: "Although this is a Sikh school, our spiritual leader is very keen that more non-Sikhs gain access to it." He said that up to 15 per cent of places could be reserved for other religions.
The school, named after the 15th-century founder of the Sikh faith, first opened as a private school in 1993 with annual fees of pound;1,400, before winning state backing in 1999. Not all pupils at the school are Sikhs - last year the head girl was a Hindu. But its popularity has led to pupils from other faiths being marginalised.
The clamour for places will be more intense in coming years after the school's management took over the running of the neighbouring Guru Nanak primary, the only other state-funded Sikh school in the country.
The primary, which was part of the secondary when both schools were private, was placed in special measures in 2003 because of concerns over teaching, leadership and high staff turnover. Senior managers from the secondary were drafted in as trouble-shooters. New staff were brought in, subject leaders were appointed and new emphasis placed on assessment. Last week the primary was officially taken out of special measures and nearly all pupils reached the national benchmark in key stage 2 tests this year.
Rajinder Sandhu, the secondary head, has now been appointed leader of both schools. Plans are also being drawn up to expand them both.