Promoting Positive Social and Mental Health Outcomes: Technology-Supported Cooperative Learning vs. Curriculum-Based SEL

School-based interventions that leverage technology to implement cooperative learning (CL) with high fidelity may improve student social and mental health outcomes. During the 2023–2024 school year, 9th grade high school students ( N = 155) in two public U.S. high schools participated in a cluster randomized trial; one high school was randomized to the intervention and one to the control group. The control school implemented a curriculum and the intervention school implemented PeerLearning.net. A three-way RM-ANOVA was used to evaluate intervention effects. The intervention had a large effect on peer support (F(1,125) = 42.91, p < 0.001, η 2 = 0.26), loneliness (F(1,125) = 21.09, p < 0.001, η 2 = 0.15), and victimization (F(1,125) = 19.02, p < 0.001, η 2 = 0.13). It had a medium effect on depression (F(1,126) = 10.29, p = 0.002, η 2 = 0.08), social stress (F(1,126) = 9.71, p = 0.002, η 2 = 0.07), and anxiety (F(1,125) = 6.71, p = 0.011, η 2 = 0.05). A small-to-medium effect emerged for bullying (F(1,125) = 4.20, p = 0.042, η 2 = 0.03). School-based interventions that leverage technology and effective pedagogy can promote positive social and mental health outcomes among high school students.