The sign on the fence, while obviously a good idea both for the safety of individuals attending athletic events at the school and for the long-term maintenance costs of the fence itself, goes largely ignored by the infected, who once again show flagrant disregard for posted signs and the rules of society in general.
The growling crowd of once-humans climb the fence en masse to chase after you, even though you clearly demonstrated the proper procedure for entering the football field by unlocking the gate, closing it behind you, and locking it again. The fact that they don’t have keys isn’t an excuse for them to put their rotting paws all over your nice clean fence. It should serve as a sign to them that they aren’t welcome in this athletic facility after school hours, but apparently the first part of the brain the virus kills is the spot where manners are.
The infected practically fall over the other edge of the fence, some of their half-rotted bones cracking as they land at odd angles on arms and legs, which clearly demonstrates the need for the sign. They then make their way, whether running, limping, or crawling prone, across the grass you meticulously maintained for years, pulling up divots and dropping clumps of coagulated blood all over the place.
You make your way to the top of the bleachers, taking shelter in the press box, the door of which is clearly marked STAFF ONLY. You don’t expect them to respect that sign any more than they did the one on the fence.