Fire safety investigators say the blaze that tore through Hawera Intermediate School on Tuesday was most likely caused by an electrical fault.
Taranaki Area Commander Pat Fitzell said the building was not broken into and motion sensors on the second floor were the first to activate.
He said there was an area of severely damaged cabling on the second level where the initial fire most likely started.
Contractors are now bringing down the worst of damaged upper level and securing the site.
Students are expected to begin classes again on Monday in a block at Hawera High School.
The intermediate school is not expected to reopen until next year.