There is no question about it and research has confirmed many times that single-sex schools and students perform better than their co-ed counterparts. It's not so much a discussion of "What if?" as it is "Why?" and really that's not even an elusive concept either.
(http://www.Singlesexschools.Org/research-singlesexvscoed.Htm) Some propaganda.
(http://www.Nea.Org/tools/17061.Htm) The National Education Association.
(http://www.Apa.Org/monitor/2011/02/coed.Aspx) The American Psychological Association.
Now of course these contain pros and cons, they are balanced resources except for the first, and if one studies this exclusively they will find that for many SSE works but for some it does not.
In today's age, school is basically the only social interaction kids get with other kids their age. Without the interaction that co-ed schools give, there's important social skills that kids may never learn. They might not learn how to interact properly with the other gender, they might develop a disliking to the opposite sex, or they could turn gay.
Co-ed schools help students learn how to interact with each other on the most humane and permissible way possible . Boys to girls and vice versa . On the backhand though , single sex schools prevents kids at a higher degree to interact with the opposite sex . And this can be a problem when getting a job or even carrying out activities in their daily lives that involves the opposite gender . Though there are other ways instead of school to help instill this kind of attitude , putting your child into co-ed school will boost the chances of getting that attitude that will land him a job and gear him up for the future .