Given a conceptual model of competencies for sustainability, such as the generic competency for sustainability established in the secondary education latest reform in Mexico, an assessment of competencies for sustainability was conducted in order to determine, in an exploratory way, the level of achievement of those competencies in the students of the main high school institution in the city of Matehuala. Within the assessment process, an analysis of the institution curriculum was required, and interactions with and teachers through interviews, focus groups, and workshops. An assessment framework was constructed in order to design an instrument that generates the learning evidence regarding those sustainability competencies and, thus, to infer their levels of achievement. The instrument consisted of a performance task, and a knowledge and attitudes questionnaire. It was applied to 60 students, divided in 3 groups according to their level of instruction. Despite the small differences among the three groups, the results showed that all students have good attitudes towards sustainability and understand the basic terms related to it; however, their performance in systemic thinking, prospective thinking and the ability to propose strategies are not developed well enough to face the challenges that the environmental and civilization crisis demand nowadays.