Re: 22 wheats and no silver?
There are a few schools of thought on this. There are places where only kids play & some towns the average income is low & adults even might look for a coin they dropped.
Active sports will help coin spillage. Is there a concessions stand? Does the school have vending machines? Helps alot if people come out of school with a purchase.
If there is nothing to spend money on there, people might not bring any.
It might seem odd to some, detecting a 1960 school when most towns in your area date to the 1800s. But I have a similar situation here, in a post-1960 suburb with 1800s central cities & some old suburbs. I found eleven silvers in a 1963 park & 3 silvers in a 1964 park. I don't drive so have hard time geting to older places & many of those have high crime.
I detected a square block park in a small Wisconsin town, got 16 wheats in an hour but no silver. Moved on to the Main Street green where they had band concerts. More wheats, still no silver.
It was 1970 or 1971 when dealers started paying 10% over face for silver, so it circulated until then.
As for various towns in your area, not every town created a park right away, but most had to establish a school within a few years. The original city plat (survey) may have established a school block. The newer school could be on the site of old playground, current playground on site of old school.
If this is the case, best results could be around the edges of the block.
The older parks will have been detected alot by people who do research, have expensive detectors & have learned then well. But if they missed anything, you might get your best find.
Reseach sites
www.maphistory.info www.hometownlocator.com, or type in your county to search engine followed by historical society or use the genealogy sites like cyndislist.com County or city websites may provide useful info. Many site ideas on Wikipedia. Best wishes, George (MN)