Fire broke out in a classroom at Auckland's Long Bay College early this morning.
Crews were called to the North Shore school's art and technology block just after 2.30am.
Forty firefighters in 15 trucks responded to the blaze and brought it under control. The fire destroyed the school's woodwork room which did not have sprinklers.
Area commander John Booth said the Fire Service had numerous calls from neighbours who heard and saw the fire burning.
"There's been quite a serious fire," he said.
"The first arriving crews ... were able to stop the fire from spreading into other classrooms."
The fire will come as yet another blow to the school, which has mourned the deaths of several students, both current and former, and a retired teacher since the start of the year.
Principal Russell Brooke said the school was closed for the day due to the risk of asbestos and to allow time for the damage to be assessed.
"[Fire crews] did an amazing job of keeping the fire just in that one building because it has two more attached to it."
Although it would not have made a difference in this fire, a new alarm system was to be installed next month.
"Every time schools build a new building the fire regulations are different so you're left with lots of rooms with different alarm systems.
"It's a costly process to manage it, it's crazy - next month we are going through the process of getting a whole new alarm system."
Mr Brooke said he is grateful no one was hurt.
The school had had lots of emails and calls of support from the community, he said.