This article presents the main results of the Programme for International Student Assessment (PISA) for Costa Rica in 2009 and then goes on to analyse the gap between the scores of students attending private schools and those attending public schools. However, the estimation of an education production function using these data shows that this gap is not entirely attributable to whether schools are public or private, but that instead the students' family environment, personal traits and, in particular, the student's grade level at the time that the PISA test was taken are all quite influential.