This paper aims to study the impact of teachers' professionalization degree on the quality of schools, measured by student performance on the standardized test saber11. We use instrumental variables (IV) models, in order to overcome possible endogeneity problems. The results indicate that the professionalization degree has a positive effect on students' performance. This effect is more noticeable in math than in language. Moreover, we find that public spending on education is positively correlated with degree of professionalization, while the adoption of the New Statute of 2002 has a negative impact.