Thanks Eldar (sorry Azarkon, didn't see that one)
The point being, boys and girls have different ways of using their bodies and body language. For some reason or other (which people which are more knowledgeable than I can elaborate on endlessly), boys seems to use strength and aggressiveness to establish their order in the hierarchy of the group. Whether it be the bully, the brightest or just the most daring. There is a competitiveness built into them that nees expression one way or the other. If left unchanneled, it leads to frustration, anger (and the dark side) and insecure human beings, because you remove an important part of their social skill training.
As Eldar said, it's not about arranging pit fights and encourage them to beat each other up, but to let them "work it out" in ways that let them channel that energy and aggressiveness in one way or other, like regular sports or other physical demanding activities.
Yes, it's probably more comfortable and easier to deal with kids if they are just sitting still, using their hands and heads (and mouths) in nice and less attention requiring ways (like not trying to see if it's possible to dig a hole through the floor).
Girls, as somebody mentioned elsewhere are often more "stealthy" (I think that was the word used), I would say devious, when settling conflicts and establishing pecking orders. The conflicts and competitiveness is still there, but as long as they all survive the day, the less visible and less damaging to the environment way of small girls behaviour appears to be more appealing to those who manage large groups of kids.