Federal Aviation Administration spokesman Jim Peters says the pilot flew over Westwood High School and its athletic fields around 6 p.m. Wednesday night.
Peters told The Record newspaper children were on the field as the pilot tossed his soggy cargo, littering the area.
There were no reports of injuries.
Peters says the Cessna 172S made three passes before landing at Caldwell airport.
The pilot, whose name was not released, was briefly taken into custody.
Peters says the man may face federal charges of reckless operation of an aircraft and dropping objects from a plane without authorization.