Suspect a bit chicken and egg... when I took the 11+ (many decades ago) there were 3 GS available for each gender - there were
-one single gender,
one mixed and
one ex technical school (single gender)
Academically they were in that order... why? well possibly because that was people tended to put them in that order on the application form, so apart from a few who wanted mixed or whose sibs went to the ex techs then the brightest went to the first then the next to the second etc.... self fulfilling prophecy.
re schools now - there are some that are known as less academic (tend to be called more rounded... maybe the lunches are good
), of course you can't try the experiment twice and it may well be that the school is very good at ensuring that all kids reach their potential - possibly a very academic school may not appreciate those who have no achieved as much and only praise the most academic ... very trying for some. ultimately it comes down to the schools - you need to chat to parents of the pupils there i think