I don't think single sex schools would teach children enough about "real life". If you only have experience of one sex, you're never going to know how to deal with the other, and that's clearly something that could be an issue of later life.
I know people say that children are distracted by the opposite sex at school, but in my own personal experience I never found that to be true at all. I found that boys and girls were able to learn alongside each other and learn key lessons from each other, too.
That's just my personal opinion, though, and I know that many others may hold other views. But from my perspective I think that girls and boys should be educated together, as there is just no reason at all to separate them.