Class war may soon break out in Canada.

More than half of Canadian high school students are in Ontario or British Columbia. Teacher negotiations continue in both provinces with secondary school class sizes a central issue that could trigger strike action.

The BC Teachers’ Federation wants to retain or lower high school class size limits reinstated by the Supreme Court after they were stripped from teacher contracts by a previous government. Ontario’s secondary teachers’ unions are pushing back against the Ford government’s plan to substantially increase class sizes in Grades 9 through 12.

To listen to the teacher unions and their supporters, the sky is about to fall. As usual, their rhetoric dwells on threats to student learning with scant attention to financial costs or pertinent research evidence. But smaller classes remain among the most expensive policy choices as more teachers must be hired or retained. This high cost must be acknowledged, particularly amid ever-growing education spending and a dearth of persuasive evidence of educational gains.

And the evidence is clear—there’s no such evidence. The extensive research literature shows small and limited benefits of smaller classes in the early grades, not high schools. In fact, there’s evidence of greater student performance in school systems with larger secondary school classes.

Consider results from a recent Fraser Institute study, based on findings from the Programme for International Student Assessment (PISA) results in 2015, the latest year of available data. The PISA project measures the performance of 15 year olds from more than 70 countries every three years. Canada has an excellent PISA record, coming 10th in math, 7th in science and 3rd in reading among the 72 participating countries in 2015. PISA also uses principal reports to measure average class sizes, yielding interesting insights.

Singapore, the top-scoring country in math and science in 2015, had eight more students (on average) in its high school classes than Canada. Japan, the top-scoring OECD county in math and science, had nine more students in high school classes.

Within Canada, there’s considerable variation among provinces, with reported average high school class sizes ranging from a low of 22.6 students in Saskatchewan to a high of 30.1 in Quebec. Ontario (24.9 students) was a little below the national average (26.4), B.C. a little above (25.4).

On this basis, the B.C. and Ontario teacher unions have little to complain about.

But the real shocker is found in the standardized test scores. Saskatchewan, with the smallest high school class size, had the lowest test scores in all three PISA subjects—reading, math and science.

Quebec, which had the largest average class size, had the highest math scores. Alberta, with the second-largest class size (28.5 students), had the highest science scores while B.C. had the highest reading scores.

And Ontario had the smallest class size—and lowest test scores in all three PISA subjects—among the four largest provinces.

Of course, these findings do not mean we can increase test scores by increasing class sizes. Many other variables are in play. But they should challenge the common misperception, held by many policymakers and parents, that smaller class sizes produce better results.

It’s not yet clear whether the B.C. or Ontario teacher unions will strike. Given their high membership fees and recent years of relative peace, both have large war chests. The Ontario unions are politically opposed to their current provincial government, the B.C. union less so.

But regardless of how the stories unfold, unions in both provinces lack credible arguments to justify striking to protect their already relatively small high school classes.