Police have said they are investigating social media post that suggests arson was the cause of the blaze at Rotorua Girls' High.
The post, allegedly written by a teenager, says: "Tonight is the night to burn down Girls' High."
Two classrooms at Rotorua Girls' High were destroyed in the blaze this morning.
A screengrab of the post was shared on Facebook, but it does not appear on the girl's page.
Northern Fire Communications shift manager Paul Madden said police received a call from the member of the public alerting them to the fire at the college on Old Taupo Rd in Rotorua at 1.20am.
"Upon arrival, the brigade found the building to be well involved in fire. It appears that there are two classrooms in a block of five that have been destroyed by fire."
Mr Madden said seven fire trucks, along with further support vehicles, battled the blaze well into the night.
The school's Board of Trustees chairman Chris Nairn said he received a phone call at 2am from a friend driving home from work. The friend told him his school was on fire.
"This is a good school, the girls will rise above it , part of life is you do get hardships but we will get through this," he said.
The Ministry of Education was aware of the fire and school was running as normal today.
The fire damaged part of H Block, the mathematics block.
Two classrooms were badly damaged by fire, while others received smoke and water damage.
A Cactus programme - a teamwork and leadership course held in conjunction with police - was held at the school this morning, but there was no power or water available, so parents brought hot breakfasts for the girls.
Rotorua Girls' High School principal Ally Gibbons said she felt sorry for the students, staff and school community.
"It's very sad for those teachers, for all the work they've put in to their learning resources," she said.
"Last night they were putting together assessments for moderation."
A reported 35 firefighters were at the scene. It's understood fire crews from Putaruru, Tirau, Ngongotaha and Mamaku attended alongside Rotorua fire staff.
The fire has been brought under control, with firefighters remaining at the scene to continue dampening down hotspots.
Roofing iron was also being moved, to allow firefighters better access to manage the hot-spots.
- with NZME.