The urgent need to address the mental health needs of students in the United States has recently gained significant attention in the aftermath of the Coronavirus pandemic and recent school shootings. Alarmingly, studies show that only one-third of the children suffering from mental health behaviors will receive mental health services, and 80% of these students receive these services in the school setting (Lin & Bratton, 2015). Unfortunately, students struggling with internalizing behaviors often go unnoticed, leading to missed opportunities for early intervention, which can have adverse lasting impacts social-emotionally and academically. School counselors, often the primary source of counseling services, play a critical role in removing obstacles to academic achievement through developmentally appropriate and evidence-based interventions. Group Adlerian Play Therapy (GAdPT) is a developmentally appropriate, feasible, and efficient intervention for engaging students simultaneously, rooted in Alfred Adler's Theory of Individual Psychology. This randomized control trial examined the impact of Group Adlerian Play Therapy (GAdPT) compared to the waitlist control group (WG) with 94 fourth and fifth-grade students who qualified with clinical levels of internalizing behaviors. Forty-eight students were randomly assigned to GAdPT (treatment group), and forty-six were assigned to WG (waitlist group). The students in the treatment group received two 30-minute play therapy sessions per week for six weeks. Results from a two (group) by two (repeated measures) split plot ANOVA indicated that compared to the WG, the GAdPT group demonstrated statistically significant improvements on (a) internalizing behaviors at home, as reported by parents, (b) internalizing behaviors in the classroom, as reported by teachers, (c) reading scores, as recorded by a standardized reading assessment, and (b) math scores, as recorded by a standardized math assessment. GAdPT demonstrated a large effect size on all measures, indicating the clinical significance of the intervention. In summary, 73% of parents and 91% of teachers reported improvements in internalizing behaviors revealing the clinical significance of the results. The standardized assessments reported that 65% of students improved in reading and 67% in math, moving from below to on-grade level from pretest to post-test. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]