This paper derives efficiency scores for around 8,500 schools in 30 countries, using Programme for International Student Assessment 2012 data and a nonparametric approach called data envelopment analysis as method. On average, achievement scores of schools can be increased by 27%, holding inputs constant. Efficiency scores vary considerably both between and within countries; the role of managerial efficiency and structural differences due to operating in different contexts (countries) is disentangled. Subsequently, a number of school-level factors are found to be correlated with efficiency scores and indicate potential directions for improving educational results. Heterogeneity of such characteristics across countries and along the distribution of efficiency is explored.