In 2002 the career of Colombian public school teachers was significantly reformed through the introduction of a selective entry competition and of further quality incentives. This paper estimates how the new quality-screened teachers impact students’ high school performance. We exploit the fact that the novel regulation applied only to newly hired teachers, whereas those al-ready in office in 2002 remained exempt, creating a mix of New-Regulation and Old-Regulation teachers in Colombian schools. Using data at the school-year-subject level, we eliminate any school-level confounders and associate the proportion of New-Regulation teachers to the variation in student test scores. We pin down a positive and significant, although not very large, effect of New Regulation teachers on student performance. New Regulation teachers have decreasing marginal returns, are more effective in larger schools and when surrounded by colleagues holding postgraduate degrees. We also document that the enforcement of the New Regulation has been somewhat unsatisfactory, since in the period 2008-2013 around 30% of all New Regulation teachers are employed in temporary positions without having passed the compulsory entry exam. These teachers have lower and less robust impacts on student performance.