And it's not just Ontario. Michele Belot and Dinand Webbink, now of the Universities of Edinburgh and Rotterdam, respectively, found that work stoppages hurt student achievement, increased the number of students repeating grades and reduced higher education attainment in Belgium. What's more, studies dealing with teacher absences for reasons other than strikes bolster these findings.
A study by Harvard's Raegen Miller, Richard Murnane and John Willett tracked the effects of teacher absences while controlling for teacher experience and skill level. They noted that teachers who are absent more regularly may be less motivated and skilled, and so they isolated absences due to poor weather, the idea being that even highly skilled teachers will be absent if the weather prevents them form getting to work.
The study found that absences lead to statistically significant drops in student math and reading scores. The drops are lower than those found in the Baker and Johnson studies, but then again, the students in the Harvard study received instruction from substitutes, whereas students in strikes get no instruction at all. Studies by Charles Clotfelder, Helen Ladd and Jacob Vigdor at Duke and by Mariesa Herrmann and Jonah Rockoff at Columbia found significant drops in student achievement because of absences in North Carolina and New York schools, respectively, with the latter finding that a lengthy absence had the same effect as replacing an average teacher with one at the 30th percentile.
The only recent study to find no significant results from teacher strikes was conducted by Harris Zwerling, a researcher at the Pennsylvania State Education Association, the state's largest teacher's union. That study compared Pennsylvania school districts that experienced strikes to those that didn't, and found no difference in outcomes once one controls for demographics and years of teacher service; this is much the same methodology as the Ontario studies. One could argue that because the study focused on U.S. schools rather than Canadian or Belgian ones, it is more directly relevant.