I've gone to both types of schools, and so has hubby. We're only two people, so take this as you will, but from what we've seen:

For boys, a single sex school means that they are less inclined to behave, because there are no girls around to call him a moron for sticking straws up his nose, belching, or lighting farts in the schoolyard. The same goes for girls, who tend to hike their skirts up and sit with their legs open when they're hot, but here's no real academic disadvantage to being more "relaxed". Unless you take it too far. And some do.

There are fewer fights in same sex schools, because there's no one to impress. People who are fans of unpopular activities, such as the Great Math And Science Club, tend to find more acceptance doing what they want, because, again, there's no one around that you need to impress. I met many more artistic personalities in the same sex school I went to than in the other ones. Mr. Anaralia, on the other hand, met more psychos.

The teachers in a same sex school are usually better paid, judging from the way they dress, so they tend to be better. But then again, that all depends.