A new classroom structure was needed, with only six usable rooms left for the school's 98 students, from Year 1 to Year 13.
Electricity supply was also affected.
"The biggest issue for us is securing the site. I'm not having kids on site if it's not secure," Mr Brockbank said.
The Westland District Council was today arranging fencing to be put up around the burned block.
Mr Brockbank, and deputy principal Nick Glancy, who is also a member of the local fire brigade, were among the first on the scene.
"It's always a surprise when you see flames like that but I have to say my immediate thought was 'thank goodness no one was here'," Mr Brockbank said.
"Luckily, a local farmer noticed it or we could have lost the whole lot."
In the light of day, emotions were catching up, he said.
"I've gone from disappointed, to angry and upset."
The block was the newest part of the school, renovated eight years ago.
- Hokitika Guardian