In the wake of the principal's move, the students at the school, located in Uddevalla in western Sweden, still haven't received their final marks for the autumn term in home economics, which left teachers fuming.
"The teacher has been prohibited from setting grades and therefore obstructed in exercising his authority," Annica Levander of Sweden's National Union of Teachers (Lärarnas riksförbund) told the local Bohusläningen newspaper.
"This compromises fairness for students and the ability of teachers to do their jobs."
According to Levander, the issue of low grades was apparent far earlier in the term, but the principal chose not to intervene.
By choosing only to act at the end of the term and by simply overruling the teachers' assessment, the principal has interfered with the teacher's authority.
"He stepped on the brakes too late," Levander said.
The teachers union has now reported the incident to the Swedish School's Inspectorate (Skolinspektionen) and the Parliamentary Ombudsman (Justitieombudsmannen).
The Swedish School's Inspectorate hasn't begun a formal investigation yet, as it is waiting for official documentation, but project manager Elisabeth Porath Sjöö explained there are limits on principals' ability to affect grading decisions.
“A principal can only get involved in the grading process if there are two teachers for a class who aren't in agreement,” she told the paper.
But, Dennis Reinhold, head of school operations at the municipality, thinks the principal acted responsibly, however.
“Based on the information I have received, I believe he made the right decision. But this is a unique situation,” he told the paper.