You're not.I can understand the risks and the liability issues. But how in Cthulu's name are you going to keep kids from fighting?
When I was in 5th grade or so, I got in to an argument with a lad in one of my classes. He got very irritated with me, then yelled "Pool doors, tonight!" then ran off.
Meaning...meet me at the doors to the school's pool (which was in an isolated wing of the building) and we'll fight. It was typically an empty challenge but not always.
This was a challenge thrown about by 10 year olds in the early 1980s. I don't think there are any fewer kids challenging one another today. If anything, there are even more of them given how some like to film themselves fighting and publish the fight on their myspace page.
And you're not going to stop backyard fight clubs, either.
But we can discourage it, and recognize that it's a pretty dangerous thing to do.