Detective Sergeant Grayson Joines told the Herald that the boy had fallen through a bubble-like skylight.
"It appears that the injured lad sat down on it... and it's just given way and the poor little fellow has gone feet up and fallen down about 25 feet [7.6m]," he said.
"He fell on to a pretty hard concrete floor with carpet."
As police arrived, the school alarm went off. The boy's mates had smashed through a door to get to him.
"The two boys were dragging their friend, who was near unconscious, out of the building," Mr Joines said.
Police gave first aid to the boy until an ambulance arrived.
Mr Joines said there was no indication that the boys were doing anything malicious or criminal at the school.
He said they were just "boys being boys" and unfortunately their fun ended terribly.