Despite a growing number of collaborative partnerships between schools and community-based organizations to expand school mental health (SMH) service capacity in the United States, there have been relatively few systematic initiatives focused on key strategies for large-scale SMH capacity building with state and local education systems. Based on a framework of ten critical factors for capacity building, as well as existing best practices, two case studies were utilized to develop a replicable capacity-building model to advance interagency SMH development. Seventy education and mental health stakeholders from two selected states participated in baseline assessments of skill competency and critical factor implementation followed by two-day trainings (one in each state); 29 (41%) of the participants also completed a six month follow-up assessment. Targeted competencies increased significantly for participants from both states, with large effect sizes (d = 2.05 and 2.56), from pre- to post-training. Participant reports of critical factor implementation increased significantly for one of the two states (t[15] = -6.40, p < 0.001, d = 1.77). Results inform specific training recommendations for stakeholders and collaborative teams, as well as policy implications to support future development of SMH service capacity.