Manchester’s King David High School has been downgraded from outstanding to inadequate after Ofsted said the way it taught its more religious pupils amounted to discrimination under equality law.

Ofsted said the separate social and educational arrangements that KDM operates in its Yavneh stream, where pupils receive more Jewish studies and boys and girls learn separately, was an “unlawful segregation on the grounds of faith and belief, and sex”.

While pupil attainment was high and teaching generally effective, inspectors identified other weaknesses within the school which was ranked outstanding just four years ago.

In a letter to parents, KDM’s long-serving chairman Joshua Rowe said the school’s lawyers believed Ofsted’s interpretation of the law was “wrong”.

The lead inspector, he wrote, had made it clear at the inspection “that if he considered the school to be in breach of the Equality Act… he would have no option but to significantly downgrade the leadership and management [and therefore, the school] to ‘inadequate’.”

But Mr Rowe questioned whether the verdict was “part of an agenda to diminish faith and religion and their associated denominational schools”, citing Ofsted chief inspector Amanda Spielman’s commitment to “muscular liberalism”.

Yavneh boys and girls have the option of being moved to the general section of the school if they choose.

But Ofsted said the narrower secular education in the Yavneh section constituted “direct discrimination” on grounds of religion under equality law.

The smaller range of GCSE subjects offered to Yavneh students was “to their detriment”, inspectors said. Smaller class sizes in Yavneh meant it was not cost-effective to offer as broad a range of GSCEs as in the general section of the school.

Yavneh generally allocated less time to music, art, drama and food technology.

There were also differences between students in the general section and Yavneh in “the length of the school day, in the arrangements for pupils to learn, eat and socialise with others and in their participation in extra-curricular activity,” Ofsted said.

Inspectors noted, too, that within Yavneh, religious education differs between boys and girls, in order to prepare them for different types of further Jewish education.

The Yavneh students “cannot freely learn and socialise with pupils of the opposite sex and so miss out on the educational and social benefits of doing this” and Yavneh girls cannot socialise with Yavneh boys at lunchtime, Ofsted said.

Yavneh students spent longer in school and “had less opportunity to benefit from the educational, physical and social benefits of leisure time. Pupils’ access to leisure time is determined by matters of faith and belief relating to these pupils and their sex.”

Yavneh girls were also denied the opportunity to take part in a production of the musical Hairspray which involved both Yavneh boys and pupils from the general section.

Ofsted also noted that Yavneh pupils took IGCSE religious studies where they learn only about Judaism whereas those in the general section take the GCSE religious studies which includes a second religion.

Some Yavneh girls would welcome the opportunity to learn more about faiths, Ofsted reported.

The proportion of students going to university had increased over time, the inspection report said, with a higher-than-average proportion going to leading institutions such as Oxbridge.

But Ofsted reported that extra money for disadvantaged students had not had sufficient impact.

While pupils reported that bullying was generally dealt with by staff, they said incidents involving racist language were not always well resolved. Two fifths of parents on Ofsted’s online survey felt the school did not deal well with bullying.

“Some parents have concerns about racism and homophobia,” Ofsted said. “The school’s records indicate that any incidents of bullying or racism are infrequent.

"Pupils told inspectors that they are not homophobic towards each other. However, older pupils and students said that the school could do more to support gay and transgender members of the school’s community.”

Mr Rowe said the Yavneh arrangements had been previously approved by the local authority and the Department for Education.

Among the options now being considered was a judicial review challenging the legal basis of Ofsted’s report or splitting the school into three different academies, he told parents.

He believed the inspection had been triggered by “a number of wide-ranging parental complaints… following the tragic death of one of our beloved pupils.

“It seems that some of the complainants believed that the school was somehow responsible. The fact is that the tragedy had nothing whatever to do with the school. Within a few hours, the inspectors concluded that virtually all the complaints were unfounded."

Despite the report, he insisted, the school remained “outstanding” and was having an “excellent year”. He said that, in a number of areas where inspectors had correctly identified the need for improvement, the school was already taking steps.

Over the past six years, King David “has had to endure savage government cuts so that its budget is now reduced by over £1.5 million per annum  - 30 per cent,” he pointed out.

“Many schools have suffered financial cuts but none have suffered as much as ours.”