Firefighters tackled a classroom blaze at Pakuranga Heights School on Sunday night.
A spokeswoman for Fire and Emergency said three appliances were called to the fire at 10.40pm.
A classroom, measuring about 20m by 10m, was well involved in flames, she said.
Three fire crews remained at the scene at 11pm.
Firefighters remained on the scene until 1.30am, the spokeswoman said this morning.
There was no indication at this stage of the cause of the fire.
* An earlier version of this story incorrectly reported the classroom fire was at Pakuranga College.