Fourth-grade students at Lafayette Elementary School in D.C. in September. (Jabin Botsford/The Washington Post)

The Washington Teachers Union clinched a long-sought victory in its fight against the D.C. Public Schools’ teacher evaluation system, albeit a minor one on procedural grounds.

An arbitrator ruled Monday that the school system had to re-hire a teacher who filed a grievance claiming he was wrongly fired under its controversial teacher evaluation system called IMPACT.

The city’s teachers union has long assailed the evaluation system, which has been central to the city’s high-profile education reform efforts during the past decade. Then-Chancellor Michelle A. Rhee introduced IMPACT in fall 2009, and critics argue that the time-consuming process is arbitrary and could result in teachers wrongly getting fired with little recourse.

The most contested aspect of the IMPACT system is that students’ test scores are taken into account when evaluating teachers. The arbitrator’s ruling in this specific case does not touch on that issue, and it does not undermine the crux of the evaluation system.

But it is the first grievance involving the evaluation system the union has won, according to Elizabeth Davis, the head of the Washington Teachers Union. In the 2015-2016 academic year alone, more than 100 grievances have been filed.

“I am hoping that this decision will become a catalyst for change,” Davis said. “We’re hoping this decision will get the school district and policymakers to rethink, and look at the tool that has been used for the last many years and see if this evaluation system has actually helped or hindered our school system.”

Thomas O’Rourke, 56, filed the grievance after he received unsatisfactory marks on his 2010-2011 evaluation and was fired from his position as a teacher at Roosevelt High School. In his complaint, O’Rourke argued that the school’s principal held contempt for him because of his union membership and alleged that the principal did not follow proper IMPACT protocol. O’Rourke said the principal observed his class for 80 minutes, when the rules call for observation sessions of 30 minutes at a time.

Teachers cannot file a grievance about their IMPACT score; they can file grievances about the protocol their evaluators follow.

The arbitrator, Charles Feigenbaum, found no evidence of any anti-union bias on the part of the principal, but he said the principal violated protocol when he observed O’Rourke’s class in excess of 30 minutes.

“I’m very pleased, over the moon. I’m glad to be vindicated,” said O’Rourke, who has been working as a substitute teacher in other jurisdictions since he was fired. “I look forward to settling this and returning to a full-time classroom teaching position.”

D.C. Public Schools suggested it would consider appealing the decision. A spokesperson said the scope of the rule was narrow and would have no ramifications on the evaluation system. The school system and O’Rourke have 45 days to “negotiate and resolve any issues relating to back-pay and benefits,” according to the ruling.

“This grievance was a singular, isolated case that has no bearing on IMPACT as an evaluation system and may not even withstand an appeal if D.C. Public Schools elects to appeal,” schools spokeswoman Michelle Lerner said.