In the present study, a teacher training program based on behavioral therapy was conducted for high school correspondence course teachers of adolescents aged between 15 and 18 years who showed developmental difficulties. Participating teachers were assigned to either an immediate treatment (IT; n = 13) or delayed treatment control (DTC; n = 17) group to evaluate the effectiveness of the program, which comprised five 90-min sessions with small groups of three to six participants and was conducted over three months. The results showed significant improvement in students’ behaviors and social responsiveness and in teachers’ confidence among those in the IT group; however, those in the DTC group did not show any such improvement. We discuss the program’s feasibility in terms of developing support resources for teachers in Japanese high schools.