Media literacy training is an urgent need of our time. Educational institutions must stand as fundamental domains to collectively address reflection on digital and media environments and prepare school-age citizens to constructively deal with the impact of the media. To do so, a paradigm shift to approach the issue is required: a critical awareness of the new scenarios created by the media and a broad reflection on their characteristics. A new framework where the spotlight is on the media, the surrounding environment is an essential reference point and training proposals are based on results and evidence. This study is part of a Design-Based Research, aimed at the creation, implementation and evaluation of a Critical Media Literacy program for high school students at the Escuela Normal Superior del Putumayo (Colombia). In this paper we present the results obtained by applying the Alfamed media competence "pre" and "post" questionnaire to the students participating in the program. The results obtained show a significant improvement both in the overall level of students' media competence and in four of the six dimensions that make up the theoretical reference model ("Technology", "Language", "Ideology and Values" and "Production and Dissemination").