Fire alarms rang out at Avonside Girls' High School at 1.15pm today, after smoke from two fires set off the sprinkler system.
"Two fires were lit in two waste paper bins in two different toilets, within about five minutes of each other," principal Catherine Law said.
All pupils were evacuated safely, along with two co-located schools, she said.
Classes were cancelled for Avonside pupils for the rest of the day, with pupils escorted back in small groups to retrieve their belongings.
"Our school evacuated, along with [Shirley Boys' High School] and of course, Ferndale School who have special needs students. So the consequences are potentially very, very serious."
Secondary consequences included the loss of learning time for pupils and water damage, she said.
A very serious message needed to be sent to pupils that there was nothing funny about lighting fires, Law said.
"Whilst we all know that young people don't necessarily do these things with the intention of malice, of hurting people, they don't necessarily think that through.
"I need every single student in our school to know that it's not fun, it's not a joke, that's not a funny thing to do - it put a lot of people's lives at risk."
The school had identified those responsible and would be conducting a full investigation, she said.
It was particularly disappointing as Canterbury was under a complete fire ban and had experienced recent wildfires in Waikari and on the Port Hills, she said.
"There's been a lot of talk about the dangers of fire in media, so it's really upsetting to think that some students would think that it's a funny thing to do, if that was indeed what they thought," Law said.
Fire and Emergency said two fire crews attended, and found on arrival the sprinklers had put out the fires.
The crews confirmed the fires were out and helped start mopping up excess water.
A spokesperson said they had since left the school.
The school areas used for sport, Polyfest and an NCEA evening were all fine and those events were continuing as planned tonight, Law said.
"In one particular area of the school, the sprinklers caused a lot of water damage to carpets and classrooms," she said.
The two bathrooms where the fires were lit, two to three classrooms and some corridors were closed, and the affected classes would be temporarily relocated for a couple of days, Law said.
But the school would be open as usual tomorrow, she said.
An assembly would be held later in the week to discuss the seriousness of the fires with pupils.
