I've read Rod-buster's story, and all the replies/comments to it so far. My thoughts:
There is no shortage of people who use school yards, after hours, right? You know, like jog the track, walk their dog, shoot hoops, etc.... Even when there's fences, there's usually always a propped open gate. And the obligatory "visitor's sign in at the office" type sign, blah blah. But like the "library" analogy: there are uses of the place that won't attract attention, and uses that will attract attention. And let's face it guys: metal detectors DO have ... uh .... "connotations" (that track joggers, and basket-ball don't have). Ie.: that you might be about to leave a mess. Right? YOU know you won't, but that doesn't stop someone eles from drawing that connotation, and griping.
But as others on this thread point out, 99% of the time you can hunt school yards. In other words, occurances such as Rod-buster's are rare. He will certainly not win the battle of semantics over whether or not he was "trespassing" or whether it's "public", blah blah blah .
So my take on this is that it's like the following scenario: Once in awhile you read in the newspaper of someone getting "roughed up" by an over-zealous cop, for a silly minor thing. Example: someone gets pulled over while driving, d/t his tire touched the yellow line. The motorist is yanked out of his car, read the riot act, put in the back of the patrol car. He's ticketed, threatened with jail, fined, etc.... All for his tire touching the yellow line. As you and I read something like that, WE IMMEDIATELY RECOGNIZE IT for being an "exception" by a single rogue cop. We do not fear driving now, nor run to "get permission to drive", etc..... right? SO TOO do I think of such stories as Rod-buster's. Sure it's no fun having to deal with authority. But in our hobby, which has admitted connotions, I'm afraid you sometimes have to get a "tough skin". And to avoid (lower traffic times) those "miss-lookie-lou's", with their cell-phones who might gripe.
You can try the alternative, of getting every last school or city bureacrat to love and sign off on your hobby, but then the downside of that is, that you can elicit "no's", which preclude you from hunting a school yard where it's entirely possible, that no one ever cared less (till your "pressing question" came to them).
As for the threat of tickets, jail, fines, etc...., you will notice that even though occasional stories like Rod-buster's *can* happen, yet .... when it's all said and done, you'll notice you rarely ever hear of anyone actually truly facing "tickets , jail, or fines" for detecting innocuous schools, parks, etc.... (where there's no rule that said "no detecting"). That's not to say that someone can't gripe, or boot you. But to the extent that it truly results in jail, tickets, etc... ? No. If you can find instances of tickets, confiscations, etc... it will be for someone sneaking obvious sensitive monuments, or someone who can't take a warning, etc......
Yes I know this "lay low and avoid lookie lous" advice will not sit well with some folks. If so, you're going to need to stick to private property, or beaches, etc...... But for turf-of-any-sort, let's face it: the man with a detector sticks out like a sore thumb.