Separation anxiety is one of the most common emotional difficulties experienced by children during the transition to formal schooling and may negatively affect academic performance, peer relationships, and psychological well-being. The present study aimed to investigate the mediating role of social adjustment in the relationship between insecure-avoidant attachment and separation anxiety among female elementary school students in Ferdows County, Iran. The study employed a descriptive-correlational design using structural equation modeling (SEM). The statistical population consisted of all female students in grades 1–3, from whom 320 participants were selected through multi-stage cluster random sampling. Data were collected using the Insecure-Avoidant Attachment Scale, the Separation Anxiety subscale of the Screen for Child Anxiety Related Emotional Disorders (SCARED), and the Social Skills Improvement System (SSIS). Data were analyzed using SPSS version 27 and AMOS version 24. The findings indicated that insecure-avoidant attachment had a significant positive effect on separation anxiety (β = 0.37, p < .001) and a significant negative effect on social adjustment (β = -0.46, p < .001). In addition, social adjustment had a significant negative effect on separation anxiety (β = -0.31, p < .001). Bootstrapping analysis further showed that social adjustment significantly mediated the relationship between insecure-avoidant attachment and separation anxiety (Indirect Effect = 0.14, p = .003). The proposed structural model demonstrated acceptable fit indices (CFI = 0.95, TLI = 0.94, RMSEA = 0.059). The results suggest that insecure-avoidant attachment is an important risk factor for separation anxiety, while social adjustment serves as a protective mechanism. Therefore, interventions aimed at improving children’s peer relationships, classroom adaptation, and social competence may help reduce anxiety symptoms among vulnerable students. The study contributes to attachment theory by highlighting the role of school-based social functioning in explaining emotional outcomes during early childhood.