Trustees voted unanimously at a board meeting Tuesday to review how students are graded after hearing from Dorval, who spoke out against no-zero marking in a brief presentation.
"This no-zero policy does not work," Dorval said. "You only have to look at the stats of the number of the high schools that have been doing this thing for a number of years and you look at their diploma results. They're terrible."
Dorval, who was recently threatened with termination by his principal at Ross Sheppard High School, said he objects to being told how to evaluate his students by "somebody who doesn't know my subject, doesn't know my students."
"The decision to make evaluations should be in the hands of teachers," Dorval told trustees. "We do take our job very seriously, consider ourselves professional, and I think it should be our right."
The board also heard from other speakers including Dorval's former colleague at Ross Sheppard, retired social studies teacher Doug Senuik.
He told trustees the no-zero directive pushes students through "by any means necessary."
The practice is not based on research, but rather the work of "overzealous principals" who want to increase their students' completion rates, Senuik said.
The practice raises marks, but lowers achievement, he said.
Tuesday's meeting followed a motion from school board trustee Michael Janz, who suggested earlier this month that the board review its grading policies.