OK if the School has not been searched then you will find most stuff around the Jungle gym and swing sets.
If the school is searched on a regular basis then you are more likely to find stuff at the pickup area and the paths leading to the play area. There is always stuff around big trees just look for the patches where the grass has been worn away.
Play area stuff is normally in the top 2 inches in searched areas and in the top 6 inches on the grass and paths. I don't dig much anymore as I have pinpointing down pat I just use a screwdriver to flip out the coin. Practice with your detector pinpointing will save hours of dig time and make less impact on the area you are searching.
Go slow and and overlap your sweeps. Try to sweep in straight lines rather than curves as you will cover the ground better. No matter how may times an area has been searched there will always be some new stuff after a few days of school.