Taieri College principal David Hunter said most pupils were not too fazed by what had happened to the 49-year-old Hislop Hall on Sunday afternoon.
''Exams were not going to be staged in there,'' Mr Hunter said.
''It hasn't been used by the students [this year].
''There will be some ex-students slightly more upset.''
Thousands of dollars worth of property, including trophies, portraits of former principals and school magazines dating as far back as the 1950s, were destroyed.
''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.
New Zealand Fire Service specialist fire investigator Mike Cahill, of Invercargill, said the fire was caused by a multibox malfunction.
Asbestos was present in the building but it was deemed ''low risk'', he said.
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.
''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 the building.''