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This study examines how the teaching staff composition with respect to certification affects student achievement in compulsory Swedish schools. We apply an instrumental variable to estimate the effect of the share of non-certified teachers on student achievement (measured by grade point average, GPA). We find statistically significant negative effects on the GPA. The effect is stronger for students with highly educated parents. A one percentage point increase in the share of non-certified teachers is expected to decrease student's GPA by, on average, 1.8 standard deviations per year. This is a substantial effect considering the large differences in the shares of non-certified teachers across schools and municipalities.