After being a teacher for over 30 years, I can tell you that some teachers (and students!!!!) can lose all of their belongings from the school door to their car. Checking sidewalk edges out to about three feet from the concrete edge, especially the downhill side is a great place to find trash and treasure. Also imagine where parents and teachers would stand while watching an outdoor event, chatting about their kids or just hanging around. Most teachers and parents in a hurry have no idea what they have lost. I found so many sets of keys and wallets in 30 years and that was without using a detector.
Turn the above around and apply it to the kids. Where would they put they backpacks down, where would they sprawl on the ground on a hot day..... and most of all, where would they play the most.
So, if you have the tear drop shaped coil on your F22, you can get about 4 to 6" of depth on low and mid conductor coin sized target in moderate to mild ground without losing too much target ID tone and number accuracy. Gold jewelry and nickels deeper than that may sound and read like pennies and dimes in your dirt. Your high conductive targets like copper pennies, dimes, quarters and silver will not be affected by this loss of target ID accuracy so much with your detector. They will still have a high tone and high numbers. So you have to decide to either go for the deep stuff (which is a questionable decision with an F22) or go for the shallower 6" or less stuff that has been missed and go for the good sounding targets. If the ground is sandy and easy to dig, anything can happen to coin sized targets. If it has lots of clay, the targets from the 1930s to 1980s may not be very deep unless there were a lot of landscaping changes.