The schools here do a good job of letting you know where they don't want you with fences and signs, but most of the playgrounds are open to the public,the sport facilities that have admission gates are normally locked tight. several have parks next door, twice I have gone from a nice quiet day in the park to a couple hundred kids on recess. I'm somewhat surprised they turn them loose into a city park. They really should have some kind of siren to warn unsuspecting park visitors. Something like they use at the dam overflow to warn fisherman, even a sign would be nice.
I treat the schools the same as parks, if there are no signs or it's not fenced off I'll hunt it but only during non school hours. The principal of one school asked me if I had permission, I told him it was allowed. He either believed me or didn't care.
One of my greatest clad days was the grass area shortcut between employee parking and the sidewalk to the entrance, one of the few times I've broke minimum wage.