Anxiety disorders are among the most prevalent psychopathologies in Western adolescents and have been on the rise in recent years. Not only does anxiety disrupt the daily lives of the young people who suffer from it, it can also have harmful behavioral, psychological, social, and academic effects. Given this, there is a pressing need to implement universal anxiety prevention programs for adolescents in one of their main living environments, the school. This type of program has the twofold advantage of reaching all adolescents before the onset of more significant symptoms and of promoting access to services for young people in difficulty. This study aimed to evaluate the effects of the HORS-PISTE--Exploration Cycle 1 program developed by the RBC Center for Academic Expertise in Mental Health, based on the initial level of problems presented by the students, using a pre-experimental pretest-posttest design. A total of 2276 students in grades 7 and 8, with an average age of 12.65 years, from 16 high schools participated in the pretest and posttest evaluations. The results showed that the students who participated in the HORS-PISTE program reported a significant change in the intended direction for several of the variables examined between T1 and T2. In particular, decreases in anxiety symptoms, fear of being judged by others, perfectionism, internet addiction, intolerance of uncertainty, and anxiety interference, as well as an increase in the sense of self-efficacy, were observed between T1 and T2 for both boys and girls. The results also showed that students who presented at T1 with more significant anxiety symptoms and proximal risk factors experienced greater improvement than did those with a moderate or low level of initial problems. Overall, the results of this evaluative study of the HORS-PISTE program are promising, demonstrating positive changes in symptoms and in the main proximal risk factors for anxiety, in particular among students who initially presented with high levels of problems. This study therefore demonstrates the relevance of implementing this type of prevention program in high schools.