ANOTHEOLDGIT, the Summary says, "The motive for discrimination was irrelevant. That the School had a religious motivation for the segregation is therefore not relevant."
It is the segregation that the Court believes has a detrimental effect and NOT the justification for that segregation.
Leaving aside the religious connotations, as I said earlier I find it difficult to understand why segregating the sexes in a mixed school is condemned (because their pupils need the interaction between the genders to prepare them for life beyond school) but single sex schools are condoned because (presumably) their pupils don't.
Although the judgement makes no comment on the reasons for the segregation or the effects it has, the Chief Inspector has somewhat different views:
"Lawyers for its chief inspector Amanda Spielman argued that the policy is unlawful and segregation leaves girls "unprepared for life in modern Britain"."
So in this particular instance it is only the girls that suffer lack of preparation for modern life. Presumably the boys get by when segregated. It is clear to me that the Chief Inspector (if not the judges) believes that this particular school is segregating the sexes and treating the girls quite differently to the boys. And that, I am afraid, is where the religious connotations need to be revisited.
There does seem to be a bit of a contradiction between segregating within a school and single-sex schools. Traditionally, girls do better when taught apart and boys don't. There is plenty of time outside school hours for socialising too though. I'm wondering what the "detrimental effect" is.
Different genders weren't segregated in either school I went to. It would have seemed unnatural, weird. That stated PT was segregated. And in primary school playground the two tended to stay apart, the boys liking rough & tumble type games, marbles, and conkers, the girls skipping, jumping over elastic band chains, and showing their knickers to the boys by doing handstands against the wall. Secondary school, the boys played football, no idea where the girls were hiding.