Ms Kemp said about 30 people had gathered on the school's back field watching the blaze.
She said the block on fire was next to the school swimming pool and surrounded on other sides by other buildings.
"It looks like [the fire's] gone through the floor. I don't think there's going to be anything left."
Former student Katie Minett said she could hear the fire "banging and cracking" from her home about 1km away.
Ms Minett said just before 9pm that the fire was starting to come under control. She said there was lots of smoke, but the flames were getting smaller.
It is understood senior staff are gathering at the school and are with police and firefighters.
A distraught woman, believed to be a teacher, could be heard at the scene talking on her phone and saying "It's your block".
One student told the Herald she, along with many others, had schoolwork inside the burning building.
Another said many of the projects are Year 13 portfolios for NCEA.
"It will be a whole year's worth of work gone so quickly," said one boy, who did not want to be named.
Student Tim Howarth said the students were talking about the fire and keeping each other updated on Facebook.
He said NCEA work from hard materials and graphics students would be lost, along with canoe polo gear.
Board of Trustees member Mark Katterns said the fire was "very devastating".
"We are really hurt that one of our old buildings has gone."
He said the technology block was about 60 years old.
Northern Fire Service shift supervisor Dallas Ramsay said about 60 firefighters were battling the blaze in the wooden building.
She said a security guard at the school phoned 111 about 7.50pm to report the fire in the 100m by 15m building. There had been no report of people inside.
Ms Ramsay said it was too early for fire fighters at the scene to determine whether the blaze was suspicious. "We're still dealing with fighting the fire."
She said there were concerns about petrol, diesel and oxy acetylene, which is used for welding, being stored in a nearby building.
Northcote College's website says the school was founded in 1877. In 2013, a Ministry of Eduction document showed the school's roll as 1,136 students.
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