For me it's down the list of determining factors. Boys might be more likely to participate in something like literature or drama without girls, but they might also be less likely to offer those activities. You also have to consider the actual class setup as in some coed schools boys and girls are separated at least for Judaics. Also, the socialization patterns outside of school matter. In my opinion it's good for boys and girls to have some time together so they realize the opposite gender is not a mystery and they understand how to have conversations and be polite.
my experience is that so much is down to the specific school that I am not sure how much I can pin on whether it's mixed or not.