Maybe we have been reading different research. Steve Biddulph argues that boys benefit from single gender classes especially in language and expression. In one school, 2 years after they separated classes for English most of the classes were achieving much higher results than previously. Apparently boys can concentrate better when girls aren't around....
I don't know if there is a perfect way of doing this. I'm on prac atm on a year 5/6 class and OMG most of the boys are little turds, but I honestly don't think that it would matter whether there were girls in the class or not. Some of them do try to 'show off' to the girls, but there are probably 2 who try to get as much approval from the boys as they do the girls. But they are the same two kids who could not give two hoots if they were at school or not and have such a disgustingly entitled attitude to everything and everyone. I'm not sure what the aim of DS1's school is by having gendered and mixed classes though and it's something I was going to ask during parent teacher interviews just for my own curiousity.