Children's social-emotional skills, such as conflict resolution and emotion regulation, have been linked to a number of highly regarded academic and social outcomes. The current study presents preliminary results from a causal test of the theory of change of RULER, a universal school-based approach to social and emotional learning (SEL). Specifically, the authors examine RULER impacts on the quality of classroom climates, the hypothesized setting-level mediating mechanism linking RULER to improved student outcomes. These analyses will provide a more precise understanding of the pathways through which RULER, a specific SEL program, contributes to children's positive development, and will also expand our understanding of the intervening mechanisms underlying effective educational interventions more generally. Data for this study come from the first randomized control trial (RCT) of The RULER Approach. The RCT took place in 62 primary schools in the Roman Catholic Diocese of Brooklyn in Brooklyn and Queens, NY. The effective sample for these analyses includes 24 classrooms (12 in each treatment condition) nested in 19 schools. At baseline, schools in the full sample ranged in size from 178 to 656 students (M = 325.92, SD = 97.06), and the number of students per teacher ranged from 17 to 35. First stage results from the instrumental variable analysis show significant intervention impacts on classroom emotional support after one year of intervention (b = 0.35, p < 0.05). Second stage results confirm that emotional support after a year of implementation has a marginally significant causal impact on classroom organization. Unexpectedly, emotional support does not have a statistically significant causal impact on instructional support. Tables are appended.