The present study investigated the longitudinal changes in social skills among junior high school students. A total of 505 students (boys = 266, girls = 239) participated in a questionnaire survey every year from the first to third year of junior high school. The results of latent growth curve model indicated a decrease in kakawari skills consisting of assertion and developing relationships with peers, and an increase in hairyo skills consisting of compassion and maintaining relationships with peers. The model also revealed a positive relation between longitudinal changes in kakawari and hairyo skills. However, the results of growth mixture model extracted a profile characterized by increase in both skills and another profile characterized by a decrease in kakawari skills only. Considering the estimated mean level and changes of both skills, we found a widening gap in social skills among students. Furthermore, multi-group analysis for the growth mixture model indicated a sex-based difference in the changes. Overall, educational practices to facilitate kakawari skills are necessary for junior high school students, particularly for girls.