"
but the poor guy got yelled at anyway for tresspassing."
Well perhaps he was being "yelled at", because no public access was allowed at that school. In other words, perhaps detecting is not the issue at all here. Firefighter has not said whether it's one of those schools that has fences up (as has unfortunately happened in this litigical age we live in).
The bottom line is, if a person is afraid of ever getting "yelled at", then perhaps this isn't the hobby for them. Heck, you can get yelled at for simply driving down the road (because someone else perceives you cut them off in traffic, or aren't going fast enough, or whatever).
You say:
"
I guess I'd rather seek permission than risk a run-in with the law and legal battles."
There is a lot of implications and issues wrapped up in this statement. Let's look at them individually:
1) If you are doing an activity, which is not specifically disallowed, then I don't see what "run in with the law and legal battles" you would incur. Again, if it is not dis-allowed, how are you in violation of something?

And no, I don't consider "destruction" "vandalism" and "damage" type clauses to apply to us. Because if WE know we will leave no trace of our presense, then that's good enough. To think otherwise, is to already have lost the battle. Because if you think you are inherently destructive or damaging, then yes, you are right. Don't go metal detecting anywhere, because you will never ever get permission to "destruct and damage" the park, anywhere in the United States.
2) Your statement implies that by gaining permission to seek such public parks and schools, will keep you free from legal hassles, yelling, etc... I can assure you this is not the case. Yes, perhaps no "legal hassles" from that one desk-bound bureacrat to who told you "go ahead". But from others? Not a chance. There have been countless stories of persons who "got permission" from city hall somewhere, and STILL get the wrath of some rank & file worker in the field. When the md'r cites his permission, the worker gets on the phone, calls to city hall (or the police dept. or to whomever gave the "permission") and gets that "permission" promptly revoked. And you, the dejected md'r, got yelled at. You may say "but at least I won't get arrested, because they .... in the short run ..... gave me permission". To that I say, re-read pt. #1. You will not get "arrested" for something that is not specifically dis-allowed. You'd have to repeatedly ignore warnings (and have failed to show them you're not "destructing") for something like this to happen, in a case where they try to morph something else to apply to you.
3) The whole premise of a statement like this, merely implies, once again, that this hobby is something that needs permission, because it is inherently evil or wrong. I still say that this would be akin to asking permission to stand on your head. Can you do it? Sure! And might a park worker say "I grant you permission to stand on your head?" Sure! And might this keep you safe from someone yelling at you, and legal hassles for standing on your head? Sure! I suppose.
Let's face it: we're in an odd hobby. The "geiger-counter" wielding guy in the park draws stares and curiousity. And true, some folk may have a knee-jerk reaction, if they see you with a lesche or screwdriver, that you are going to be a nuisance, leaving messes. No amount of "permission" is going to allay that. My hunch is, in that city where that park worker laughed at you for even asking, would probably never have even noticed or cared, if he'd just passed you by. But contrastly, there are a lot of people who have tried this, and gotten "no's", where no one ever cared before. Now guess what's going to happen, when that person who fielded such an inquiry, sees other people md'ing in the future? They're going to remember the earlier inquiry, and start booting others! I've seen this happen before. Moral of the story? :P