UPDATED 2.42pm: The 12-year-old boy who fell through a roof skylight at a Palmerston North school last night plunged about seven metres and suffered serious head and internal injuries.

He remains in a serious condition in the Starship children's hospital in Auckland, to where he was airlifted at 5am today.

The youngster was one of a group of boys who were seen on the roof of the Palmerston North Central Normal School in Featherston St just before 8.30pm.

A concerned neighbour called police and they arrived to find that he had fallen through the skylight.

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 onto 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.

Police have spoken to the injured boy's friends and will contact the 12-year-old and his family when he is in a better condition.

His family are with him in hospital and are being supported by the police and Victim Support.

Central Normal School principal Shona Oliver told the Herald she could not comment on the incident as police were investigating.

She did not yet know the identity of the injured boy or whether he was one of her school's pupils.

"For us, it's just very sad for a family at this time of year," she said.

The Fire Service attended the incident to assist police.