But it was not caused by someone forgetting to pay the power bill.
Staff and pupils chose to flick off all the switches at the Dunedin secondary school so they could experience education without electricity, just like their great-grandparents did.
Principal Rick Geerlofs said the main purpose of the day was to spread awareness of environmental issues, such as coal burning, in support of World Environment Day, June 5.
"No Power Day is to demonstrate to students, staff and our community that we can find other ways to learn without lights, power, computers, projectors and heating.
"We also hope that as a result of this day, we all develop a better awareness of the amount of electricity and coal we use, and a greater awareness of opportunities to reduce consumption."
Although the school intended to run classes the whole day without heating, mercy was shown and it was turned on in some places around the school because of the snowy and frosty weather.
Year 7 pupil Jasmine Henderson said on a typical school day in winter, she would wear a jersey.
But yesterday, she came prepared.
"I had a jacket, a jersey, several tops and a blanket.
"I'm glad I did it. It would have been freezing without it."
Teaching staff went to great efforts to adapt their teaching for the day, to make it fun and educational for pupils.
Years 7 and 8 dean Damian Burden said he missed being able to use technology, such as his data projector, but the lack of light and heating was easily surmountable.
He supplied glass jars with candles in them to provide light, and moved desks in the classroom closer to the windows so pupils could be warmed by sunlight.
"It kind of reminds you of what it was like to teach without technology."
College executive officer Brian Lee estimated the day had saved the school about half its daily power bill.
