The aim of this paper is to analyse the students' performance in the mathematical competency aspect of the Programme for International Student Assessment (PISA) 2015 tests and to compare 5 Spanish regions in this respect -- Navarra, Castile-León and Catalonia, with results above the national average; and Extremadura and Andalusia, with results below the national average -- in order to identify the factors causing these differences. To do so, we computed the degree of association between variables related to the students and to the schools with the scores obtained in the mathematical tests. The purpose of this analysis was to better understand the meaning of these scores and their causes and, above all, to propose educational policy actions for improving the students' mathematical performance. In this study, a 2-level regression model was applied to the data collected in the PISA 2015 tests. The first level included the factors related to the students and the second was composed of the variables related to the schools. Our results highlight the significant influence of factors such as immigrant status, grade repetition, location of the school (rural or urban) and economic and sociocultural status. The relevance of these factors to students' academic performance has been observed in previous editions of the PISA tests. We emphasise the need for action to improve students' mathematical performance and, therefore, their educational success.