Background: Mental-health-related stigma prevents active help seeking and therefore early therapeutic approaches and the recovery of functionality. National and international agencies recommend the implementation of prevention and mental health promotion programs that support the elimination of stigma in the classroom, since most mental health problems usually start in the adolescent stage. In view of the evidence that teachers present stigmatizing attitudes towards mental health, it has been considered as convenient to carry out an anti-stigma program with the main objective of evaluating the impact of an intervention based on the education and promotion of mental health, aimed at teachers and counsellors of a secondary school. The specific objectives were to get to know which were the most stigmatising attitudes that prevailed in the sample before and after the intervention; to evaluate the knowledge of the teaching staff and counsellors on psychosis before the intervention; to analyse correlations between clinically relevant variables; and assess whether this programme was beneficial and feasible for alphabetising counsellors/teachers of educational centres on stigma and FEP. Methods: This was a non-randomised clinical trial in which a nursing intervention was performed. Tools: a psychosis test (pre), Stigma Attribution Questionnaire (AQ-27) (pre-post), and satisfaction survey (post) were used. The inferential analysis included the Wilcoxon and the Pearson Correlation Test. Results: In the sample (n = 22), the predominant stigmatising attitude was “Help”. The p-values obtained in the Wilcoxon Test were statistically significant, except for “Responsibility” and “Pity”. The following constructs of interest were faced: “Fear”–“Age” and “Professional experience”; and “Help”–“Psychosis test”. Conclusions: Despite the scores obtained in “Responsibility” and “Pity”, the intervention was useful for reducing stigma in the sample. Implications for the profession: There are adolescents who have suffered stigma from their teachers, and consequently have minimized their symptoms and not asked for help. For this reason, we implemented a nursing intervention based on the education and promotion of mental health, with the aim of expanding knowledge and reducing stigma. In fact, this intervention, which we carried out on high school teachers, managed to reduce the majority of stigmatizing attitudes measured on the stigma attribution scale.