I believe a large amount of people form relationship and social skills with the opposite sex while in school and if you are only in a single-sex school it's very hard to do that and that disadvantages them in life later on when they want to form a relationship with the opposite sex since they are inexperienced.
I already think it is a bad idea to break kids up by age groups. This gives people the idea that we're all defined by groups right away, and that you cannot relate or speak evenly with someone out of your age group. Separating genders makes this even worse. It's terrible.
Yes, I would certainly consider a student who attended a single-sex school to be disadvantaged. Single-sex schools are leftover relics from our Victorian past and are no longer relevant in today's world. Any student who attends a single-sex school will not have learned how to mix socially with both sexes, he or she will not have had an opportunity to develop friendships with the opposite sex, and therefore would be at a distinct disadvantage.
I do not believe that a student who attends a single-sex school would be at a disadvantage compared to those who attened a coed school. I think that it varies on the situation and people in terms of how it affects them. But there is nothing to suggest that either case is an advantage or not.