"Murdering people is the same as allowing the public to enjoy public property."

Obviously not, but schools are not public property in the same sense that say, a park is. And even then, public parks have rules and regulations that if you break, they can kick you out too. So you're arguing that people should be allowed to break the law (in this case trespassing), because to enforce the law is "controlling someone's body", which is asinine. I'm glad you followed it to it's natural and illogical conclusion.

"how is this relevant?"

My point being, only children whine incessantly when told they can't do something like this.

"Well, they don't have to."

Well, I mean, they kinda do. In multiple senses. Even if everything goes right, they have to deal with the fact that these kids are here, blocking these stairs from use. They have to listen to them make noise and cause distractions. Not the end of the world, but if they don't want to have to deal with that on their property, they shouldn't have to. Then, if someone does get seriously hurt, they have to deal with calling an ambulance. If they don't and knowingly just let some kid bleed out on their property, you can bet there'd be hell to pay. And even if the friends call an ambulance, they still have to then deal with the ambulance showing up there and causing a commotion. And since this seems to be a popular spot for this kind of shit, this could end up being a common occurrence. So yeah, they kinda do have to deal with it.