Why would a public school have a no tresping sign?

I drove past my old middle school and there were big signs on the sides of the buildings that said POSTED - NO TRESPING. NO SKATEBOARDING, SKATING, BICYCLING, ETC. VIOLATORS WILL BE PROSECUTED. These signs were never up there before until now. Why would a PUBLIC school be off limits? It is not private property. Public schools are paid for by our tax dollars. I used to go up through the courtyard every now and then just to see all the things that changed at the school after all those years. There must be some reason why a public school has made the campus off-limits?