The parents hired an attorney and sued because there wasn’t a sign on the glass door telling students that they could harm themselves if they punch the glass.
It seems there would have to be signs on a great number of solid objects (walls, desks, floors, doors, people, trees), if the default is it’s presumed ok to punch something unless there’s signage signifying it’s unpunchability. You would also have to have signs on the signs warning you to not punch the signs
Actually this is easy to solve. You put up a sign at the gate which reads “Everything assumed hazardous and non-punchable unless labelled otherwise.” Then you hand out hazard warnings for all the students and staff to label themselves.