The Chicken Coop Key is located in the Dumpster area of the School. It is in the deepest part of that area, guarded by some Ball of Ears Spirits and a Child Spirit. To get it, you need to figure out the Ball of Ears Spirits' pattern of movement and get pass them without running to avoid them detecting you. When you reach the deepest part. keep the flashlight flashing on the Child Spirit while getting the key in order to prevent the Spirit from killing you.
This key is used to unlock the Chicken Coop, which contains the Pool Key.