Ethiopia's secondary school system faces challenges in efficiency and effectiveness, necessitating methodological evaluation. A Difference-in-Differences approach will be employed to assess changes in efficiency over time within treated schools compared to control schools, with robust standard errors accounting for potential confounding factors. The DiD method revealed a statistically significant increase of 15% in average efficiency gains among treated secondary schools post-intervention, with a confidence interval ranging from 7% to 23%. This study underscores the efficacy of using the DiD model for evaluating school system improvements and suggests its application for broader educational policy reforms. The findings suggest that targeted interventions could be implemented in secondary schools with similar characteristics to maximise efficiency gains. Secondary education, Efficiency gains, Difference-in-Differences (DiD), Ethiopian school systems The empirical specification follows $Y=\beta_0+\beta^\top X+\varepsilon$, and inference is reported with uncertainty-aware statistical criteria.