Two people are critically injured following an explosion in the boiler room of Orewa College, near Auckland.
St John Ambulance says one person suffered burns to up to 90% of their body, while another person suffered head injuries. Both are in Auckland Hospital.
The Fire Service says the incident happened about 2.40pm on Wednesday and a fire was contained.
Principal Kate Shevland told Checkpoint a staff member and a contractor were fixing a problem with the boiler which malfunctioned on Tuesday.
Ms Shevland says a loud explosion was heard throughout the school and steam was seen coming from the boiler room.
Matt, a 14-year-old student, says there was confusion as students and staff tried to work out what had happened. He says his teacher evacuated his class to a rugby field once the fire alarm started ringing.
Orewa College, in Rodney District, has a roll of about 1600 students. The school will be closed on Thursday and counselling is being organised for students and staff.