And i agree about the "bitchiness" stuff being a red herring.
My primary had bitchiness a-plenty. Always about boys. The girls would fight each other over boys by my last year in primary. Utterly depressing.
My friend attended a mixed school and she said that the girls were forever patrolling each others' sexuality -"lesbian", "slag", "he's mine", - and either falling out or physically fighting - yuk.
There was, indeed, an undercurrent of that at the g. s. but nowhere near as extreme.
For all the talk of how civilising mixed schools are, I sometimes think they are not that great for girls. Unless you have a very strong-willed girl, they really can become a machine for inculcating a fembot view of womanhood, as well as enforcing a very narrow, very conservative identity and sexuality.
But you know, that was then ... . And surely it depends on the ethos of the school and the children it attracts?