Schools are encouraged to take a proactive approach to mental health through the use of a tiered system of supports facilitated by universal mental health screening. However, schools may experience difficulty implementing universal screening and large-scale systems change. The purpose of this study was to explore multilevel implementation determinants, identify strategies that aided implementation, and examine how implementation determinants and strategies impacted implementation reach for two schools adopting and initially implementing universal screening within a tiered mental health system. We used a mixed-methods case study design where qualitative analysis of focus groups identified themes related to implementation determinants and strategies, and quantitative data compared implementation reach before and after the initial implementation of universal screening. Focus groups identified themes of broader influence, structures, school climate, personnel, and resources that enabled and obstructed the implementation process. To aid the tiered system, schools used implementation strategies of outreach and systems, screening, and intervention planning. Integrative mixed-methods analysis identified influences of these determinants and strategies on changes in implementation reach at Tier 1; however, only minimal changes occurred at Tiers 2 and 3. Results highlight the importance of attending to implementation factors when adopting and initially implementing a tiered mental health system.