An explosion at Orewa College north of Auckland this afternoon left two workmen critically injured.
Firefighters say it appears a boiler exploded at about 3pm.
Two men with critical injuries, suffering burns and head injuries, were helicoptered from the scene.
Deputy Principal Mark Quigley said the school's senior groundsman, called Rick, was one of the injured men.
A third contractor was uninjured.
Students were evacuated but none were hurt. One eye witness described the explosion as "extremely loud".
"The whole school shook. The roof of the room it was in lifted. Thick glass windows smashed completely to pieces."
Mr Quigley said there had been a "steam explosion in the boiler room."
"There had been a problem in our boiler room last night and the caretaker and a contractor were in the room doing maintenance."
Principal Kate Shevland said tonight staff and pupils heard a loud explosion and could see steam coming from the boiler room.
Three school staff had rushed to the aid of the injured while all school pupils were evacuated, he said.
Sergeant Greg Foster said the injured men were suffering from burns and head injuries.
Three fire engines from Silverdale and East Coast Bays attended, along with three ambulances, police and a Labour Department representative.
A Fire Service spokesman said firefighters put out a fire, which was described as not major.
Occupational Safety and Health are investigating.
The school is closed tomorrow and possibly Friday.
Ms Shevland said counselling would be available for students who witnessed the incident and for staff members involved in administering first aid.
"We're all very upset that two people have been very seriously injured as a result of this," she said.