Firefighters arrived at the school around 7:15 p.m. and found a transformer near the edge of the school's property putting out smoke and nearby residents without power.
PG&E spokeswoman Tamar Sarkissian said nearly 400 homes were affected by the blackout, caused by the malfunctioning transformer. Sarkissian said most had power restored by 8:30 p.m. and all but a handful were back up a short time later.
Greenbrook officials determined that the transformer posed no threat to students and chose to keep school open Friday morning with the understanding that power would be restored at the earliest possibility.
Sarkissian said crews worked all through the night replacing linkages in the transformer to ensure the smooth flow of power, but when teachers began arriving Friday, classrooms were still dark.
Parents were notified of the outage by e-mail, and were told to have their students dress warmly as there would be no heat until power was restored. Teachers held class in their darkened rooms, and children took flashlights with them to the restrooms.
Chartwell's, the company that provides food for the school's cafeteria, was unable to cook so hot pizzas were brought in to feed the students who normally purchase their lunches.
Sarkissian said power began flowing to the school shortly before noon, although with systems being reset and tested, it was a short while later before all function was restored.