The aim of this work is twofold: first, to find out if there are differences by gender and academic level in relaxation and mindfulness competence in high school students. Second, it is intended to analyze the relation between Relaxation-Mindfulness, Classroom Climate and Academic Performance based on the hypothesis that classroom climate acts as a mediator between relaxation-mindfulness and academic performance. The study was conducted during an intervention of TREVA’s Relaxation-Mindfulness Program on a sample of 420 students of Compulsory Secondary School and High School with an average age of 14.29. The results showed no differences by gender in mindfulness but partially, by educational level. It was found that classroom climate acts as a mediator between mindfulness and academic performance. The implementation of programs of relaxation-mindfulness is highly recommended for Secondary and High School students.El objetivo de este trabajo es, por una parte, averiguar si existen diferencias por género y nivel académico en la competencia en relajación y mindfulness en el alumnado de Educación Secundaria Obligatoria y Bachillerato. Por otra, se pretende analizar la relación existente entre relajación-mindfulness (REMIND), clima de aula (CA) y rendimiento académico (RA) partiendo de la hipótesis de que el clima de aula ejerce de mediador. El estudio se realizó durante una intervención del Programa TREVA de REMIND sobre una muestra de 420 alumnos/as de Educación Secundaria Obligatoria y Bachillerato con una media de edad de 14.29. Los resultados no muestran diferencias en REMIND por género pero sí, parcialmente, por nivel educativo. Se comprobó que el CA ejerce como variable mediadora entre REMIND y RA. Se recomienda la implementación de programas de relajación-mindfulness en los centros educativos de Secundaria y Bachillerato.