This study utilized data from the Korean Children and Youth Panel Survey 2018 (KCYPS 2018) to examine factors influencing academic engagement among third-year middle school students and to investigate differences in pathways across levels of academic achievement (high, middle, low) using structural equation modeling (SEM). Focusing on family background factors (parental SES and parent–child relationship), student–teacher relationships, and the dynamic concept of self, a two-step approach was employed, consisting of measurement and structural model validation, followed by multi-group analysis. The results indicated that parental SES had direct effects only on the parent–child relationship and academic engagement, while the parent–child relationship influenced academic engagement indirectly through the student–teacher relationship. The student–teacher relationship emerged as the strongest predictor of both academic engagement and self (I) across all groups. Indirect pathways mediated by self (I) and self (II) were significant only in the high-achievement group, with a specific differential pathway—parent–child relationship → student–teacher relationship → self (I) → self (II) → academic engagement—observed. These findings suggest that the influence of parental and teacher relationships on students’ self-development and academic motivation varies according to achievement level. Accordingly, the study highlights that enhancing the quality of interactions with parents and teachers can serve as a key mechanism for promoting adolescents’ self-development and academic engagement. By demonstrating that middle school students’ academic engagement is shaped not only by structural factors but also by the self formed through relational experiences, The study offers policy and practice implications for promoting reflective, supportive interactions between parents and teachers that enhance students’ self-development and academic engagement.