The objective of this study is to explain the role of informal student network structures in the reproduction of educational inequality among upper secondary school students in Tehran. This study adopted a mixed-methods design. The statistical population consisted of upper secondary school students in Tehran, from which 420 students were selected using multistage cluster sampling. Quantitative data were collected using a researcher-developed questionnaire measuring informal network structure, social capital, access to informal educational resources, perceived educational inequality, and academic performance. Additionally, 28 semi-structured interviews were conducted with students, teachers, and school counselors. Quantitative data were analyzed using descriptive statistics, Pearson correlation, and multiple regression analysis, while qualitative data were analyzed through thematic analysis. Results indicated that informal network structure, network social capital, access to informal educational resources, and family socio-economic status significantly predicted educational inequality and jointly explained 47% of its variance. Access to informal educational resources was the strongest predictor. Stronger network connections were associated with higher academic performance, whereas weaker network positions were linked to greater educational inequality. The findings demonstrate that informal student networks function as a powerful hidden mechanism in the reproduction of educational inequality, suggesting that educational policies must address these social processes to achieve sustainable equity.