Is single sex education illegal now, or is there something missing from this article?

[ThisIsTheEndgame]

www.bbc.co.uk/news/uk-england-leicestershire-48446674

Can someone explain to me why it is illegal to segregate by sex within a school when we have entire schools that exclude one or the other? (Fwiw I'm in favour of people having the option of single sex schools).

I can’t see why it would be illegal on the face of it, unless one group is getting a different curriculum/facilities/some other substandard education as a result. Which is perfectly possible if they do not have two sets of teachers, one of each gender.

In most single sex schools you will still meet people of the opposite gender, reading between the lines perhaps the segregation is particularly strict?