Thousands of dollars worth of property, including trophies, portraits of former principals and school magazines dating as far back as the 1950s, were destroyed by Sunday's fire at Taieri College.
The fire in the school hall was caused by an electrical fault, a fire investigator said.
Principal David Hunter said the 49-year-old Hislop Hall was being used to store sports and administration property and documents while a new block was being built.
He said property stored in the hall was due to be moved to the new block yesterday and senior management staff had been working out of the building while construction took place.
''There were some archives in there and they were, ironically, going to be shifted today,'' Mr Hunter said.
''The biggest thing is what happens now.
''Only time will tell when we're going to have a usable space again.''
The fire did extensive damage to the inside of the building but structural damage was still unknown.
Pupils of the school were ''not devastated'' about the building but some former pupils might be upset, Mr Hunter said.
''Exams were not going to be staged in there. It hasn't been used by the students [this year].''
Fire investigators were on site yesterday establishing the cause of the blaze.
New Zealand Fire Service specialist fire investigator Mike Cahill, of Invercargill, said it was not suspicious and was caused by a multibox malfunction.
He could not say how old the multibox was or whether it was overloaded.
''[The fire] was quite smoky and quite intense. The damage is pretty consistent for at least half of the building.''
The heat would have been about 800degC at its hottest, Mr Cahill said.
Asbestos was present in the building but it was deemed ''low risk'', he said.
Firefighters from seven brigades extinguished the blaze and nine appliances attended, from Dunedin, Mosgiel, Roslyn, Lookout Point, St Kilda, Willowbank and Outram, about 3.10pm.
An aerial appliance and the command unit were also deployed.
Ministry of Education head of education infrastructure services Kim Shannon said in a statement the ministry had contacted Taieri College and would continue to work with them.
''We have engaged an emergency building contractor to ensure the building is isolated and made safe, and they have been on site.
''Once we know the extent of damage and likely cost of repair, we will be able to help the school make decisions about the future of the building. We may have to consider replacing [it],'' she said.
