This study aims to identify the main determinants of students' performances in reading and maths across eight European Union countries (i.e. Austria, Croatia, Germany, Hungary, Italy, Portugal, Slovakia, and Slovenia). Based on student level data from the OECD-PISA 2018 survey and by means of the application of efficient algorithms, we highlight that the number of books at home or a variable combining the type and location of school represent the most important predictors of the students' performance in all the analysed countries, while other school characteristics are rarely relevant. Econometric results show that students attending vocational schools perform significantly worse than those in general schools. Looking at differences between students attending schools in big cities and those in small cities, they are never statistically significant except in Portugal. Through the Gelbach decomposition method, which allows to measure the relative importance of observable characteristics to explain a gap, we show that the differences in test scores between big and small cities depend on the schools' characteristics, while the differences between general and vocational schools are mainly explained by the families' social status. Results appear robust to the hierarchical model approach.