Poor posture has been shown to decrease both visceral and respiratory/circulatory function as well as to increase neuro-musculoskeletal system stress. Improper postures of children at school and in daily life can affect their physical and psychological development. In particular, many children who refuse to go to school or who have experienced school refusal have physical and mental problems. Given that posture is closely related to one’s psychological state, modifying one’s posture can improve both physical and psychological health problems. This study examined the changes to school-refusing students’ physical and psychological condition after attending an intervention to improve their posture. The participants were 65 high school students who have experienced school refusal and were attending a program to modify their posture for 2 months. Their posture and psychological states were recorded both pre- and post-intervention with the following measurements: postural alignment and spinal curve according to a sagittal plane, the General Health Questionnaire 30 (GHQ), and the Subjective Adjustment Scale (SAS). Post-intervention, most of the participants saw improvement in their postural alignment (e.g., trunk inclination in standing position, P  < 0.001, 95%CI 2.00, 4.00). Participants with improved standing posture post-intervention had higher SAS scores (e.g., feeling of acceptance and trust, P  < 0.05, 95%CI −3.00, −0.00). We found that easy-to-implement postural interventions have a positive effect on students’ mental health. Furthermore, it was suggested that their adjustment to school would also improve as their posture improved. The contribution of this study shows that it is possible to care for the physical and mental health of students without using special facilities and techniques. It is hoped that the findings of this study will lead to an improved adjustment to both school or novel environments, as well as prevent health-based school refusal.