Zero Tolerance policies are the refuge of imbecilic administrators and bureaucrats who don't want to take on the responsibility of making real judgements. Instead they rely on cookie cutter rules, regulations and policies. When someone questions them on the implementation of the policy in a certain case they get to wash their hands of it and say "My hands are tied, it's the policy."
As far as whether or not children can be legally bound by any contractual agreement...unless their parents signed the policy, the children ARE NOT legally bound by it. Minors CANNOT, by statute and law enter into binding contractual agreements. They are subject to laws and policies though. The signature on the paper means nothing however.
All that is beside the point though, because zero tolerance policies are bogus, and when anyone actually tries to defend them in cases such as this or in cases like the high school honor student who was expelled and will not be allowed to graduate because someone spotted a couple of STEAK KNIVES left over from a camping trip in the back of his pick up truck, the idiocy of them becomes patently obvious.
Violence is different from play. Violence occurs when one individual is subjected to the threat of direct injury. Psychological violence is more difficult to ascertain, but potentiality cannot enter into it. Violence does, or does not occur. If the boys did not harm one another or any other child, then violence did not occur.
Like I;ve said before: when I was in grammar school in the seventies, we used to routinely beat the hell out of one another at recess and walk away at the end of recess like best buddies. Was it violent? Sure, was it a crime? Hell no. No offense was truly given or taken. We all entered into the brawl freely and essentially for recreation. It was more like an unorganized boxing and wrestling club. The only time anyone got into trouble was when blood was drawn.
If these liberal dolts have their way, in another generation or so, the US will not be able to field a credible army because even justifiable violence will be socially bred out of us.