ingles2 - in answer to your question "why?"
The first reason both DSs went to a girl heavy nursery, Pre-Prep, Prep was because we were lucky to have that available in the places we have lived and they were well regarded institutions so it was not as if we were picking poor schools.
Having identified the opportunity, we then made a positve choice in that direction because we liked the calm focussed thoughtful atmosphere of the girl heavy schools. A stereotype but it was true in those girl heavy schools compared to the 50:50 mixed ones we looked at.
We did experiment last year sending the DSs to a mixed 60:40 boy:girl Prep but it was a disaster. Both suffered academically and often came home unfocussed, unhappy and with their minds in chaos. We grabbed at the chance to move them back to a girl heavy mixed school that had just gone mixed this year. Indeed, if they had been the only boys in the school we would stil have done it.
By the way our boys are 'typical boys' but what is really noticeable is how orderly their minds are at the girl heavy school. I know it is a stereotype but the atmosphere is very noticeably orderly, polite and responsible at their current girl heavy school whereas at the boy heavy school last year it was noisy, frantic and a lot of mean physically violent behavour.
Our DS1 (9) tends to play with girls who are interested in his complicated adventure games while DS2 just does not think about it and plays wth anyone. They both hardly played with girls at the boy heavy school last year because any boy who played with girls were immediately called 'sissy' or 'gay' or 'in love'. The girls also stayed away from boys as they were frightened of the violent play.
It is a wholly more positive atmosphere at the girl heavy school this year. The girls are even quite physical with the boys and certainly stand their ground in arguements and debate. Great stuff!