The purpose of this study was to examine the efficacy of an abbreviated, classroom-based, teacher-taught Acceptance and Commitment Therapy (ACT) program as an intervention to improve mental health in adolescents. In a group-randomized controlled trial, students (N = 586, age 14-21) were nested within 34 classes, which were in turn nested within 14 schools. Individual classes were randomly assigned to either a four-session ACT program or a usual-curriculum control condition. Students were assessed using questionnaires at pre- and post-treatment, and a 12-month follow-up. Questionnaires assessed quality of life, internalizing and externalizing problems, thought and attention problems, and psychological inflexibility. Hierarchical linear modeling showed no significant improvements on any of the outcome measures compared with the control group. No substantive effect sizes for ACT across time were observed. These findings failed to support ACT in the format that was used in this current study, which was as an abbreviated, classroom-based, teacher-taught program to improve mental health for all students. We had a large sample and many outcome variables, but failed to find any statistically significant effects or substantive effect sizes. In this study, ACT was delivered by teachers as opposed to mental health professionals, so it is possible that professionally trained therapists are needed for ACT to be efficacious.