She said the two-story technology block looked as though it would be "completely gutted" by the fire.
View: Fire at Northcote College
"The roof has fallen in and all the windows have blown out," she said last night.
"It's all completely collapsed."
Former student Katie Minett said she could hear the fire "banging and cracking" from her home about 1km away.
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.
Board of Trustees member Mark Katterns said the fire was "very devastating".
"We are really hurt that one of our old buildings has gone."
The technology block was about 60 years old, he said.
About 60 firefighters had battled the blaze in the wooden building, Northern Fire Service shift supervisor Dallas Ramsay said last night.
A security guard at the school phoned 111 about 7.50pm to report the fire in the 100m by 15m building, she said. There had been no reports of people inside.
This morning the Fire Service said a skeleton crew was manning the building, and had been on stand-by over night.
A fire investigation team would examine the scene today to determine the cause of the fire.
