Very heavily debated topic. I tend to believe that statistical trend because it is consistent with my experiences.
The fact of the matter is most only will experience one form of private school education. I am in a rare position as I have the insight of attending both an elite GPS school, and a co-ed school. The common trend regarding the GPS school was dissatisfaction amongst parents. However, I have yet to hear a negative word about Barker, from students or teachers alike. Could this be more of a reflection on the school itself? Yes, but I tend to think a co-ed environment heavily contributes to the positive environment of the school.
From experience, I think the "we specialise in teaching boys" rhetoric is a scam. I don't think these schools achieve adequate character development, because I don't think these environments effectively discipline boys. Also, the lack of female interaction will always mean a severe lack in social education.
Time will tell, but I am 100% convinced both Newington and Cranbrook will become far healthier environments. This is especially positive for Cranbrook who seem to have a recurring "scandal" appearing amongst their boys. I imagine this "scandal" will become far less prevalent.
I do understand this is a rugby forum though, and I do believe Cranbrook and Newington's rugby program will suffer. My one "complaint," (and I use that word loosely) about co-ed education, is a noticed a significant drop off on the competitive nature of boys, however, I definitely don't think that's an objective negative.
Open up your minds, don't get left behind.