The Longitudinal Influence of Mothers’ Co-parenting on School Adjustment of Left-Behind Children with Absent Fathers in China: The Mediating Role of Parent–Child Attachment

School adjustment of left-behind children (LBC) is a significant public health issue that may affect their well-being and mental health. In China, most left-behind children have a family structure in which the father goes out to work and the mother stays at home to take care of the children. As the sole caregiver in the family, a mother who shows support or destruction to the father’s parenting goals or behaviors in the process of raising children may affect the LBC’s school adjustment. However, the relationship between mothers’ co-parenting and LBC’s school adjustment and its mechanisms are unclear. This study aimed to explore the longitudinal influence of mothers’ co-parenting on school adjustment of father-absent LBC, and the mediating roles of paternal and maternal attachment. 284 father-absent LBC (177 boys and 107 girls) aged 8–11 (Mage = 8.43, SDage = 0.73) from Anhui Province completed questionnaires about mothers’ co-parenting, paternal and maternal attachment, and children’s school adjustment at two time. The results showed that (1) mothers’ positive co-parenting positively predicted LBC’s social competence, and mothers’ negative co-parenting positively predicted LBC’s antisocial behaviors; (2) Maternal attachment mediated the relationship between mothers’ co-parenting and LBC’s social competence, while paternal attachment mediated the association between mothers’ positive co-parenting and LBC’s antisocial behaviors. This study reveals the effect of mothers’ co-parenting on the school adjustment of father-absent LBC, and the unique mediating role of paternal and maternal attachment. Moreover, this research suggested that encouraging mothers to exhibit more positive co-parenting behaviors, reducing conflict and disparaging fathers, may be an effective way to promote LBC’s school adjustment.