I would sent DD to a girls school in a heartbeat if I could but I cannot find a similarly good boys option for DS so in the spirit of fairness they will go to a mixed school.
I've never understood the principle of depriving one child of what would be best for them, in some kind of misguided 'fairness'. My dc know that we will always do what is best for each of them, individually, whenever a choice is offered. We are fair to all of them, by doing the best for all of them, not 'sacrificing' one of them because another might not get the same opportunity.
I think that's a good point by JustGotToKeepOnKeepingOn , about maturity and the difference between boys and girls.
The sole reason I’ve done this for them {single sex schools} is so they can enjoy and concentrate on education and not have to navigate dealing with boys.
See, None of my dc have ever had to "navigate dealing with boys" as none of them see the other sex as a separate species. My ds has always had plenty of friends who are girls and my dds have always had plenty of friends who are boys. Friends are friends. I think in keeping boys away from your dds, if anything, you could be creating a situation where 'navigating' has to happen, rather then just being mates like people who grow up together.