Staff at New Plymouth Girls' High are devastated the school's performing arts building has been extensively damaged by fire, its principal says.
About 24 fire officers including volunteers from Waitara were called to the blaze about 10.45pm on Sunday and spent most of the night putting it out.
The building could house up to 100 people for drama performances and is metres away from a new school block.
Principal Jenny Ellis says the inside looks gutted and there is a huge amount of smoke damage.
She says fortunately, a cupboard containing an extensive collection of costumes is sealed off and may have been spared.
Mrs Ellis says the building was well-used and the school is known for its performing arts programme.
The Fire Service says so far it does not appear that the blaze was deliberately lit.