Physics teacher Lynden Dorval told the Canadian Broadcasting Corp. he couldn't follow the "no-zero" policy at Ross Sheppard High School out of good conscience.
The policy was adopted a year and a half ago based on the premise uncompleted assignments and exams reflect behavior and not ability, the report said.
Instead of a zero grade, teachers are supposed to write a remark instead and give grades on whatever work was completed, which Dorval said he couldn't follow.
"I just didn't have a choice," he said. "I tried to talk myself out of it [giving zeroes] many times, but it was just something so important to me, I just had to go through with it."
Students are given chances to make up missed assignments, and Dorval said most of students did.
The teacher said most parents aren't aware of the school's policy as teachers were told not to publicize it.
Edmonton Public Schools issued a statement saying, "This is a staff discipline issue and we can't speak to the specifics of this individual case."