Two young women have been charged with arson, over a fire at Sacred Heart Girls College in Hamilton.
The fire broke out just before midnight on Friday. Two classrooms were damaged.
Detective Sergeant Terri Wilson said there was substantial damage to the second story of the building.
"It consists of a staff room and then adjacent classrooms on that level and then the classrooms directly below where the seat of fire was, also damaged, which will cause disruption for the remainder of the year in terms of schooling.
Mr Wilson said the two 19-year-olds would appear in Hamilton District Court on Wednesday.
Detective Sergeant Terri Wilson said members of the public provided information which assisted the police inquiry.