Underfunded secondary schools in Indonesia rely heavily on limited BOS (School Operational Assistance) funds, yet administrative staff often lack adequate financial management capacity, threatening transparency and accountability. Participatory training offers a promising, cost-efficient capacity-building approach. This study examines the effect of a participatory training program on financial management knowledge, self-efficacy, and report accuracy among administrative staff in underfunded secondary schools, while also exploring participants’ subjective experiences. Methods: A mixed-method explanatory sequential design was employed, beginning with a one-group pretest-posttest quantitative phase (n=40 administrative staff from ten underfunded secondary schools) followed by qualitative semi-structured interviews with 15 purposively selected participants. The three-day participatory training incorporated simulations, case studies, role-play, and group discussions. Instruments included a knowledge test (α=0.85), a financial self-efficacy scale (α=0.89), and a financial report accuracy checklist (inter-rater κ=0.82). Data were analyzed using paired t-tests, Cohen’s d, and thematic analysis. Significant improvements were found in knowledge (t=12.34, p<0.001, d=2.01), self-efficacy (t=11.22, p<0.001, d=1.89), and report accuracy (t=9.87, p<0.001, d=1.64). Qualitative findings revealed three major themes: enhanced conceptual understanding, increased confidence and motivation, and improved collaborative practices. Participatory training effectively strengthens financial management capacity in resource-constrained school settings, enhancing both technical competence and professional agency. Integration of participatory approaches into continuous professional development policies is strongly recommended.