The second fire to damage part of Richmond School in just one month is being treated as suspicious by police.
Emergency services were called to the Napier school at 1.40pm on Saturday, after a fire started on a fence and spread to a single-storey building.
Two fire engines were needed at the scene and the fire was under control by 2.30pm.
Principal, Shane Foster said it was "disappointing" but was glad there was no "serious" damage and that no one had been hurt.
"The community reacted pretty fast in terms of notifying the fire brigade, because they got there before some serious damage, otherwise we could have lost everything."
Security footage identified three children, whose names are now with police, Foster said. However, they are unknown to the principal.
"In the footage, we saw them arrive and then we saw them leave and as we saw them leave, we saw the smoke come out of the building.
The damaged building, called the Whanau Resource Room is used to house sports uniforms and equipment.
Despite no damage inside the room, Foster said they needed to make sure the uniforms had no smoke damage.
Today insurance assessors are expected to evaluate the damage.
It is understood about one-third of the roof was burnt, something which need to be fixed as soon as possible, Foster said.
However, they are also looking at ways to prevent another incident.
"We'll have to get together as a board and discuss whether we have lock our gates in the weekend at night when we leave to make sure no one comes into our school grounds."