BBC News reports that Islamic single-sex education has been ruled unlawful. Yet there are more than 400 single-sex schools in the UK. Why is the Islamic one unlawful when hundreds of single-sex schools exist in the UK?
The background to the case is that the school received an unfavourable report from Ofsted. The school board successfully applied for a judicial review of the report, asking it to remove references to a breach of the Equality Act 2010. Ofsted appealed this decision and the appeal was upheld.
Some of the relevant sections in the (now released) report are:
Boys and girls are segregated in lessons and at social times from Year 5 onwards in this state-funded mixed-gender school. Boys and girls are taught on separate corridors, have separate breaktimes, are not allowed to mix during the shared lunch hour and have limited opportunities to work or learn together. Leaders have ensured that both boys and girls have access to the same curriculum and facilities. However, the decision to organise the school in this way limits pupils’ social development and the extent to which they are prepared to interact with the opposite sex when they leave school.
Leaders say that the decision to segregate is faith-based because their interpretation of Islam discourages mixing of genders for this age group. However, the school’s policies and practice do not consider how to mitigate the potentially negative impact of this practice on pupils’ chances to develop as socially confident individuals with peers from the opposite gender. This is an issue not addressed in previous inspections, but it does not accord with fundamental British values and amounts to unlawful discrimination.
The appeal judges' view was that although boys and girls were treated comparably, they could still be subject to unlawful discrimination as individuals.
43) Ofsted's case on the first three grounds of appeal is that the Judge made a mistake of law in approaching the issue of discrimination by comparing the girls, as a group, with the boys, as a group, rather than looking at the matter from the perspective of an individual pupil
56) ...we do not accept the School's argument that, whether looked at from an individual or group perspective, separate but equal treatment by reason of gender cannot be unlawful discrimination even if it is detrimental.
As James K noted, there is an exemption to the Equality Act for the purpose of admissions for single-sex schools. The evidence isn't really clear on whether single-sex schools provide better results academically: there are hints that it does but there are too many confounding factors to be sure.
The Equality Act was passed by a Conservative government, which generally favours parental choice in education. It would be politically damaging to take this choice away, especially as the then Prime Minister, David Cameron, attended a single-sex school himself. This would be only something that a radical progressive party would likely attempt.
Segregated religious schools, being relatively few in number, and relatively recent, don't have the support of centuries of tradition. Either they didn't have enough lobbying power, or failed to foresee how the Equality Act would be interpreted.
When they have segregation as a desirable religious goal, promoting social confidence with people from the opposite gender is going to be hard. With traditional single-sex schools, there will often be links with other schools, shared senior years etc., so even without an explicit exemption, they could make a better case for lack of harm.
Footnote: US readers may be interested to know that the court considered the arguments in Brown v Board of Education, although didn't find the analogy compelling.
Why is the Islamic one unlawful when hundreds of single-sex schools exist in the UK?
The answer seems to be in the first article you link to:
Birmingham Islamic faith school guilty of sex discrimination [...]
Ofsted's lawyers argued the segregation left girls "unprepared for life in modern Britain".
This isn't limited to Islam, one might add. Per the same article:
About 20 schools - Islamic, Jewish and Christian - are thought to have similar segregation policies.
Best I'm aware other UK schools are secular and dispense the same education in all schools. The real issue is religious vs secular schools, with sex discrimination occurring in one but not the other. Or at least less often and less blatantly.