In general the Sex Discrimination Act 1975 does not apply to single sex schools. The discrimination by the school against the few boys in th gymn club (which is separate from the school) prima facie to be lawful. It is, it seems, on purely economic grounds.

Interesting, in a sort of way, that most UK single sex private schools ("public schools" in UK speak) were established by or for one religions, over time came to contain a larger proportion than the population as a whole of another religion, and now, it seems, contain a larger proportion than the population as a whole of a third.