The paper joins the efforts of other scholars in investigating secondary education efficiency by applying a non-parametric methodology. In this respect, the paper’s purpose is to review some previous researches on measuring the efficiency of public (secondary) education sector as well as some conceptual and methodological issues of a non-parametric approach. Most importantly, the Data Envelopment Analysis (DEA) technique is presented and then applied to a wide range of EU and OECD countries, with a special focus on Slovenia and Croatia, to evaluate the technical efficiency of secondary education. The empirical results show that technical efficiency in secondary education varies significantly across the great majority of EU and OECD countries. Many EU countries, including Slovenia and Croatia, show a relatively high level of technical inefficiency in their secondary education as they respectively only rank in the last two quartiles among selected countries. Therefore, taking advantage of the significant room to rationalise public secondary education spending without sacrificing while also redirecting resources to the tertiary education sector is recommended for both countries.