There is growing evidence that knowledge and skills acquired in compulsory school have lasting effects into adulthood (Gustafsson, 2016), and that the first years of schooling are particularly important for students’ later educational trajectories. This raises the question of which early school determinants are most influential for student achievement. Among school resources, class size has been shown to affect achievement (Fredriksson et al., 2013), although effect sizes are modest. In contrast, several syntheses conclude that teaching and teachers are of greater importance (Hattie, 2009). Longitudinal studies consistently demonstrate substantial differences in student outcomes across teachers (e.g., Nye et al., 2004), with teacher-level variation accounting for approximately 10 percent of the variance in achievement. At the same time, evidence linking observed teacher characteristics such as education and experience to student outcomes has been mixed, partly due to methodological limitations and imprecise definitions of teacher qualifications (Blömeke & Olsen, 2019). Nevertheless, several studies report positive associations between teacher quality indicators and student achievement (Wayne & Youngs, 2003; Johansson et al., 2023). There is, moreover, a need for robust evidence, as much previous research relies on single-country studies or cross-sectional data. Building on this literature, the present study examines variation in teacher effectiveness within schools and tests the hypothesis that teachers with relevant teacher education and longer experience have a greater impact on student learning than teachers without such qualifications. This study addresses these gaps by utilizing the complete longitudinal dataset from the TIMSS Longitudinal project. The analysis encompasses all participating countries across two cohorts: a primary cohort (Grade 4 to 5) across nine countries, and a lower secondary cohort (Grade 8 to 9) across three of these countries. Using two-level regression analyses (students nested within classes), the study investigates the impact of teacher quality indicators, such as teacher education and experience, on mathematics achievement gains. In addition, the study examines the relationship between student socioeconomic status and teacher quality to assess whether teacher quality is equitably distributed. We hypothesize that the distribution of teacher quality varies across countries as a function of the compensatory mechanisms of the school system. These findings provide evidence on the role of teacher quality in promoting student achievement and shed light on social selection effects.

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