Ok, my turn for a different take on this matter. Now, as a disclaimer, I did not read ALL of the posts on here- it got too lengthy and they all started to read about the same. I am going to approach this from my own background and experiences which many feel conflict with MDing (higher academic and political).
1. Blend. MDing is, to put it mildly, controversial in the eye of Joe Public. It is a close cousin to the popularized deep-sea treasure hunting. To some it is thievery. To others we are digging holes and making a mess. But to most, it can be a form of jealous awareness- wanting whatever it is we are digging up, and you know it MUST be gold coins that guy just dug out of the playground. Dont detect when there is a ball game going on. You may as well be chasing a ball in traffic. You will get nailed. And it will hurt. A lot.
2. Get written permission if possible from your friend/ ex teach. Written permission is better than oral permission is better than no permission is better than a no. Know who to ask and how. In the future, when you are confronted with an imbecile like the fellow who spouted off about untold treasures under his playgrounds, thank him for his time and walk away. Then go next door. Dont come back later and ask his wife which is pretty much what you are thinking about doing here at a board meeting. What do you think the person he goes to bed with is going to say? And pissing him off more wont win her over either. Get the analogy? Now, you still have permission to hunt another property. Go hit that one. Preferably when someone isnt planting flowers or for petes sake not while there is a school picnic or something going on.
3. The law is not on your side. Despite your luck living someplace where there are not extensive rules or laws preventing you from detecting on public property, that can change faster than a fart. The second you go to your town board or whatever and have it on the agenda, its public and printed in the local paper. Suddenly, instead of a little private matter, Joe Public is going to pop in and raise some concerns. And ANY little mess some past MDer left that anyone can remember will be brought up in spades. It doesnt even have to have happened in your county. Could be a mess on a beach in Tahiti in 1970 while Ernest and Olivia were on their honeymoon or a simple swingers getaway. Before you know it, a conversation is begun and a decision will be postponed until next month when it will be put on the agenda again for more public input and some official research done. They may even appoint a committee. How grand! See where this is going? And you just wanted to win your little pissing contest.
4. Share. Instead of circumventing the Supergoof and involving the school board, why dont you approach the local historical society and get them involved with your "project" (see more on this word in number 5). If you work with them and are willing to give them some of the really cool historical finds- assuming you find some (like old school tags, etc.) they may be very interested in what you find and will support you. You could even put on a display for them during an open house or historical event in the town. Or go through them to contact the local 6th grade teachers to see if they would like to do something with the kids for the history aspect of it, possibly do a seeded dig and have the kids work to identify things and the history behind them. This brings the community together and puts a very nice spotlight on you and the hobby in the eyes of Joe Public. If it works, make sure the paper is involved too. It all helps.
5. Get a thesaurus. No matter what path you choose to take, even if you think Im bonkers and you still want to confront the board, your word choice will make all the difference. Its amazing how fast walls are put up in people's minds and ears as soon as the wrong words come out. Instead of a hobby, how about a project? Projects have goals and often can have an educational slant. Instead digging (which can sound more like 'excavating' to untrained ears), tell them you are pinpointing targets and pulling them out. Then hold out the handful of pop tops, torn aluminum and rusted iron and tell them that cleaning up the area also keeps the little kids from getting cuts in their hands. 'Can you imagine the lawsuits if one of these ended up piercing a toddler's shoe and into their foot??' You dont have to tell them the rusted nail was 6" down. Which brings me to my last point.
6. Dont tell Joe Public everything. For one thing, they dont really WANT to know every little detail anyways. The official looking folks, some with badges, just want to know you are not going to make the place look like the fairway on Caddyshack. They also want to make sure you are not robbing the place of either history or history-making hoards of gold. Keep the reality to describing how many cups of coffee you can buy at the local lunch counter with the change you find each morning. Then tell them its a relaxing way to get away from your wife or husband for a couple hours. Think of it as going fishing but not caring if you catch fish. 'Please dont take that little slice of heaven away from me!'.
Above all, avoid confrontation. But when Joe Public does come around, invite them to take a look. It takes away the mystique, and puts a friendly face on the weird guy with the headphones on. Keep it real. Keep it honest. And keep it going. Good luck!