Flickr user jenny downing
Not the substance in question, but just to get the concept going.
Update August 14: They know it's not asbestos, but still not what it was. Check out the follow story at The Island Packet.
First reporting August 13: Found in six rooms around the floor of sink cabinets, a mystery white substance has raised concerns and prompted testing for anything toxic at Galer Elementary School at Laurel Bay.
No definitive answers yet, but you can head over to The Island Packet for a rundown on the situation.