if you look at gcse education league tables in general single sex schools perform better then mixed however there are a few on the top of the list. another factor is environement most of these schools are in areas where parents are highly eduated. please refer to bbc.co.uk they have league tables or the time newspaper. i have not read any material on the point you mentioned if you have the authors of the research i would appreciate that.
There are some american researches on this subject.I will try to find some papers.
It is interesting that single sex schools perform better,but we need to compare students of equal IQ and motivation.If parents are higly educated,they probably have an IQ above 120,so their children are in advantage and their differnece in performance due to single school environment is difficult to establish.
We need to measure average IQ in the single school classes and then compare their gcse performance to that of a random sample of mix school classes,whose students have average IQ similar to that of single school classes.If the number of students attending single schools is above a certain number,the results can be statistically significant.It is also important to measure performances in separate subjects like female performance in maths in single schools compared to female performance in maths in mix schools and compare the average as well the low and high performances,what percentage they make.
I am sure there have been studies about that because it is a hot pedagogic issue.