This article and its Online Supplemental Material describe the development of the California School Wellness Index (CSWI), a tool to aid mental health research and practice in schools, specifically for wellness screening and monitoring. The CSWI is a 0-40 composite index derived from responses to the Brief Multidimensional Student Life Satisfaction Scale (BMSLSS) and the Social Emotional Distress Scale (SEDS) and rooted in the Dual-Factor Mental Health model. The development of the CSWI involved psychometric and normative analyses using data from 626,940 California secondary students during the 2021/22 and 2022/23 academic years. Analyses conducted with other independent samples examined concurrent validity with the Social Emotional Health Survey--Secondary (N = 78,769, collected in 2020/21) and stability and predictive validity with the Mental Health Continuum-Short Form (N = 1,828, collected in 2020/21 and 2021/22). Distribution and ranking information and clinical interpretation resources for the BMSLSS, SEDS, and CSWI are described. The discussion and Online Supplemental Material provide insights into strategies for using the CSWI within a Multi-Tiered Support System (MTSS) for various research and clinical applications. These applications include using it as a population-level global mental health index, identifying students needing more support, communicating with educators, guardians, and community members about student well-being, and evaluating the effectiveness of student support services. The Online Supplemental Material includes development, validation, and interpretation information and open-access resources for using the CSWI across MTSS levels.