No one yet knows how many Jewish schools could be affected by last week’s judgment that it is unlawful to keep boys and girls apart in a co-educational school.

Ofsted believes the number of Muslim, Jewish and Christian schools potentially in the frame amounts to the “low 20s”, after the Appeal Court ruled that Al-Hijrah — a state-aided Muslim school in Birmingham which teaches children from four to 16 — breached the law by segregating the sexes.

Already, one Jewish school, Hasmonean High in London, which operates boys’ and girls’ divisions on separate sites, has indicated it might have to split formally into two schools.

A spokesman for the Department for Education, welcoming the Appeal Court judgment, said it “supports the department’s long-standing position that mixed schools should only separate children by gender in very limited circumstances where this can be justified and they can demonstrate that no pupil is disadvantaged by virtue of their gender.”

The three judges (who all happen to be Jewish) found Al-Hijrah’s practice of “complete segregation” breached equality law by denying boys and girls the opportunity to interact with one another.

Amanda Spielman, chief inspector of Ofsted, which brought the appeal following an earlier High Court ruling in favour of the school, said Al-Hijrah was “teaching boys and girls entirely separately, making them walk down separate corridors and keeping them apart at all times. This is discrimination and is wrong. It places these boys and girls at a disadvantage for life beyond the classroom and the workplace, and fails to prepare them for life in modern Britain.”

The question is now where the authorities intend to draw the line.

Hasmonean, an outstanding academy, has taught boys and girls at separate locations for more than half a century but technically represents a single school.

State-aided primaries such as Menorah in Golders Green (another outstanding school) and Yesoiday Hatorah in Prestwich both teach boys and girls in gender groups. Beit Shvidler in Edgware has separate classes for Jewish studies. Another state-aided school, King David High in Manchester, has a more religious stream, Yavneh, where boys and girls learn separately.

OYY, an independent Lubavitch school which received a positive Ofsted report earlier this year, teaches boys and girls on different sites.

Rabbi David Meyer, executive director of Partnerships for Jewish Schools, had a point when he observed it was “very difficult to find an educational argument that can justify the existence of some separate sex schools but render others illegal. Sadly the outcome of this case is likely to be more about limiting parental choice than raising educational standards.”

Designated single-sex schools are permitted to exist under the law.Still, if the judgment stands, it may be that schools will be able to comply with the law with some bureaucratic finessing.

Hasmonean executive teacher Andrew McClusky notified parents it might have to consider separating “into two separate single sex schools within one multi-academy trust, but this will need to be clarified”.

The ruling nevertheless gives the school an additional headache it could do without. Hasmonean currently has plans to redevelop the girls’ site in Mill Hill and relocate the boys from their rundown premises in Hendon to a separate building next door to the girls. But it is already having to overcome objections to the plans from the Mayor of London over green-belt considerations. Before any redevelopment can go ahead, it is likely the legal status of the school will also now have to be sorted out.

What this judgment indicates is the creeping power of equality law. And there could be further challenges down the road.

The judgment didn’t say separating girls and boys for some activities amounted to discrimination in itself. Traditionally, the ecclesiastical authorities of a faith school have enjoyed considerable freedom in organising its religious studies and some Orthodox Jewish schools run different programmes for boys and girls in Jewish studies.

But the law seems to be edging towards a point where if a judge felt that girls educationally lost out by having a different Jewish curriculum — for example by being denied the opportunity to learn Talmud — he or she might feel there are grounds to intervene.