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 an area of severely damaged cabling on the second floor showed where the fire was likely to have started.
He said early indicators had ruled out arson as a probable cause and contractors are now bringing down the worst of the damaged upper level and making the area safe.
The school's students are set to begin classes again on Monday at Hawera High School.
The intermediate is not expected to reopen until next year.