Concerning over 90% of the coins being pennies, some thoughts/questions:
Is it an elementary school? Were you detecting near the building or does the school have sports fields? Is there an outdoor concessions stand, or vending machines inside? Do the kids pay for lunch with coins?
There are many areas kids go that adults don't. Or kids are doing the type of activities that more likely result in coin loses that what adults would be doing.
Pennies may be more likely thrown away than quarters, which are more likely to be accidental losses.
Inflation: Parents may be less likely to give their kids money. Or if vending machines are in school a 12 oz soda pop may have gone up from 75c to $1, so no change. Or a 20 oz soda pop may have gone from $1.50 to $1.75 or $2
Is the income of the townspeople relatively low? You can compare household incomes to nearby towns at
www.city-data.com
A badly corroded penny on some detectors seems too wide in pinpoint to be a coin, more like a can, so may be undug.
Are most of the pennies zinc (1st made in '82)? Many detectorists don't bother to dig them. Best wishes, George (MN)