Ashburton College. Photo: RNZ / Cole Eastham-Farrelly
A social media post said students were sent home about 10am on Monday due to a lack of heating, lighting and internet.
Students who used school buses were asked to remain at the school if they could find another way home.
"We thank everyone for their understanding in this unexpected event," the school said.
Electricity Ashburton general manager Pete Armstrong confirmed the outage was limited to the school and did not affect the wider network.
