There are actually two issues here to consider, and I think many have touched upon them: learning styles and socialization.
There is quite a bit of research that shows there are learning style differences between boys and girls. Some good links are below. The first also includes information around studies done proving the information provided. The second, though primarily informational, also gives some biological differences in how boy and girls brains differ.
http://www.genderdifferences.org/research-learning.htm
http://www.nsba.org/site/doc.asp?TRACKID=&VID=2&CID=91&DID=5637
Some of the more important findings identify the following:
1) Girls tend to learn better in a contextual and cooperative learning evironment; boys do better in a confrontive, competitive one.
2) Girls need more building up with their confidence; boys need more reality checking.
3) Girls are better auditiorily and verbally; boys are better visually and spacially.
These are just some, but highlight some of the differences.
Having worked with middle and high schoolers for more than 15 years, the above examples mirror some of the observations that I have made. Often, depending on the teacher and teacher's style/personality, boys or girls will excell or do poorly because of that teacher's style/personality rather than the students intelligence. So, from a purely learning style/educational viewpoint, single sex schools are advantageous.
Socialization is a whole other matter. There are two schools of thought here. The first is that a same sex school eliminates the inter-sex competition for the attention of the opposite sex, reduces the distractions caused by trying to impress, helps increase intersex cooperation, and allows both boys and girls to particpate in activities that might be embaressing if the opposite sex were in school. The other is that a same sex school does not mirror real life. All of these previously mentioned issues need to be learned and so seperating the sexes does not allow for this.
Reasearch done by the US Education Department shows that in both areas of educational performance and socioemotive develpement, same sex school tend to do better than co-educational schools, though their are two caveats, in my opinion with the research. Firstly, the same sex schools are universally private schools, which research also shows that students do better in those areas then students in public schools. This automatically adds a variable to the research not accounted for, contaminating the data. Secondly, equal amounts of research shows that there is no difference in perfomance between same sex and co-ed schools.
Link to the above mentioned research:
http://www.ed.gov/rschstat/eval/other/single-sex/index.html
PDF of the entire study:
http://www.ed.gov/rschstat/eval/other/single-sex/single-sex.pdf
I tend to favor a blended school environment. Most of the day is same sex (prinaril the educational portion), but certain social daily activities would be co-ed. This allows for both the advantages of a same sex learning evironment and for the important socialization of a co-ed environment.
If pressed for a choice between the two, however, I would go with the co-ed environment. Even though the research is contrary to that position (though I question the validity of the research for the reasons I mentioned), it is my position that learning how to cope with different learning styles and to navigate different social situations are important skills that help to prepare our youth for the world after school. At a new job, very few will train you, taking your learning style into account. There are very few single sex workplaces, and understanding how to deal with the opposite sex in cooperative and competitive situations is important for both males and females to learn.
I do think that our co-ed schools can be doing more to assist our youth both in learning the social skills important to navigate the world and to both teach towards specific learning styles and educate these students in what their learning style is. I think doing this would change some of the research data in favor of these co-ed schools.
Some information used in this post was obtained here:
http://privateschool.about.com/cs/choosingaschool/a/singlesex.htm