Children and adolescents from multicultural families with an immigrant mother tend to experience stress due to problems with cross-cultural adaptation in South Korea, which has a homogeneous cultural identity. This study explored the impact of acculturative stress on externalizing behavior problems (e.g., aggressiveness and delinquency) among 121 elementary, middle, and high school students in Gyeonggi. It analyzed the mediating effects of acculturative stress on externalizing behaviors using structural equation modeling with bootstrapping method. The results showed that stress experienced in relation to cultural adaptation indirectly affected externalizing behaviors through school maladaptation. This indicates that while acculturative stress itself does not directly affect the externalizing behaviors of children and adolescents with immigrant mothers, it does appear to increase the difficulty of adjusting to the school environment, and as the maladaptation worsens, externalizing behaviors may subsequently develop. Schools must function as a protective environment against externalizing behavior problems associated with acculturative stress.