The purpose of this study was to examine the changes in peer relationships and class cohesiveness spurred by a school group art therapy program in upper-grade elementary school students. Twenty-seven students were randomly selected out of six fifth-grade classes at S Elementary School in G City. A single-subject research design was employed. A school group art therapy program was conducted for 80 minutes, once or twice per week, 12 times in total including pre- and post-test. The Index of Peer Relationship, kinetic school drawing(KSD), and class cohesiveness examination were used for program assessment instruments. A paired t-test was conducted as the pre- and post-test of the study group to examine the significance of the score changes for peer relationships and class cohesiveness. The projective test of KSD was quantitatively compared to examine the pre- and post-test. The results of the study were as follows. First, positive peer relationships increased, and negative peer relationships decreased in the subjects, suggesting significant changes. Second, school atmosphere, mutual communication, and mutual trust of class cohesiveness showed significant results. Morale and unity increased, but the increase did not show significant results. Third, the school art therapy program effectively improved the indicators of interaction level found in school life. This study suggests that the school art therapy program improves peer relationship and class cohesiveness and is effective to early detect and prevent school maladaptation elements in upper grade students in elementary school students.