I can't really read the whole thread in the time I have, so sorry if someone already said this, but
Both. There are serious pro's and con's for both.
As you said in the first post, the distraction of the other gender can really drop a students academic performance. I can't focus with all the girl that are in my class, makes my mind stray from the subject of whatever we are talking about in class, and it really does get annoying. Sometimes when I try to focus, I can't. At all. I try to, but that makes my mind wander even more. When you have only one sex in a class room, it can be less distracting, making kids focus more on their academics.
The biggest con is that school isn't just for learning. School is also for making close ties with people who you are always around, and without the other sex to interact with every day, kids really don't get all of the social interactions that they need to stay healthy. Many problems can develop with someone who doesn't have someone other then their closest same sex friends to talk to. Kids need a different view of the world, and that is what the other gender does for them.

I am not sure, but I  can say that there are probably ups and downs for both. As rcarter said, there would be less distractions if the sexes were separated, but then again, in the real world you need to have that social interaction, and that is what school is supposed to prepare you for. If you were just completely cut off from the other gender, then it would just be really awkward and different, and dating is already a stressful and difficult topic to broach, and it would only be all the more so if it was just one sex in a classroom. I am not really sure though, on one side you are more focused academically and you are bound to perform better and get better grades on your work and on exams, but the downside is, you lose that social interaction with the opposite sex. On the other hand, you are more distracted and are also more likely to not pay attention during class and perform worse, but you have that interaction. It could go either way for me though...I wish there was an option that satisfied both sides personally. 

What the..... I did not even know that exists.  i mean i have a few females as friends (No not girlfriends). I don't do worse at school because of that. Trust me. It's medieval that is what it is. It's discrimination. What if a boy does not feel good at girls or what if a girl does not feel good at boys. What about if you're gay. Or if you're lesbian? So i say single sex classrooms are ridiculous and are bad.

You should do both. That way children can learn to interact with both boys and girls. For example, I'm always joking with my friends (who are boys) but I can also have a lot of fun when I talk with the girls in my class. If you divide the genders, you might get bored soon or you want to be serious with someone else from the opposite gender,  or you might want to have a lot of fun with someone else from the opposite gender. So dividing genders is not very good for your social education I think. The only class where it isn't that bad though is Gym Class, because at some point boys start to get stronger and faster than girls.