<p>This study explores how student-centered learning (SCL) and student-teacher relationships (STR) influence mathematics achievement among secondary students in the International Baccalaureate (IB) program in South Korea. Drawing on data from 520 students enrolled in the Jeju IB Education Longitudinal Study, this research applies Structural Equation Modeling (SEM) to examine the mediating roles of intrinsic motivation and learning effort and the moderating role of private tutoring. The results show that SCL significantly predicts mathematics achievement both directly and indirectly, primarily through its positive effects on intrinsic motivation and learning effort. Learning effort emerged as the strongest predictor of achievement, confirming its pivotal role in translating motivational and instructional inputs into academic outcomes. Surprisingly, STR did not show significant associations with motivation, effort, or achievement, suggesting that in high-stakes, inquiry-driven environments like IB mathematics, instructional quality may outweigh the influence of interpersonal teacher-student dynamics. Multi-group analysis revealed important differences based on private tutoring participation. Students receiving private lessons responded more strongly to student-centered instruction, likely due to greater content familiarity. In contrast, students without tutoring relied more heavily on intrinsic motivation to drive their learning effort. These findings emphasize the need for differentiated instructional strategies that support both tutored and non-tutored students. The study contributes to discussions on reducing reliance on private education by demonstrating that genuine classroom engagement, not external tutoring, is key to academic success in the IB context. It also highlights the IB curriculum’s potential to promote equitable learning when paired with student-centered teaching practices.</p>