I live in a smallish town, too, and I hunt where I will. Schools are off limits (as they should be) til the weekend or holidays, but I have never asked - nor been bothered. Actually, Ill take that back. Once, a very long time ago, a guy came out from the school and asked me what I was doing. I knew the principal - he put his attittude away when he heard this. But that is the ONLY time I have ever been confronted. Now, I just hit em on the off school days and never look up.
Parks are public access, for recreation of said public. The only consideration is if the municipality has restrictions, permitting, etc in place. So, learn the rules that will hurt you by calling the municipal code office in your jurisdiction. Simply ask, if there are rules about metal detecting, nothing more. Many towns have none and they usually just say, "Huh?"
As to parks hunting, if kids show up I talk to the parents if possible. If lots of people move in, I move on. If someone "official" confronts me, we talk it out - which has never happened. I always wave first and smile long before that. I also wear a Hi-Viz highway workers vest when at a park, so I look like I belong there.
But, to make it simple, here's what I suggest: Just go to it. Most people dont give a hoot, even the groundsmen, as a rule. On this one, play dumb, bud. Its easier to get forgiveness than permission.