The population’s tendency towards the acquisition of unhealthy lifestyles, especially during adolescence, is a problem of increasing concern nowadays. Having a good breakfast, eating five pieces of fruit a day, engaging in physical activity and reducing the amount of pastry and sweets one eats are just some of the habits covered in this paper. A six-month-long project was conducted to improve healthy living habits and physical activity for students in their fourth (and last) year of compulsory secondary school (with a final sample of 52 individuals). This was the dependent variable, while the independent variable was the methodology proposed in the Decalogo Saludable (Ten Commandments for Good Health). The methodology’s main characteristics were that it called for schools and families to work together and it fostered a feeling of responsibility in students. In fact, the health habits encouraged were never ordered by the teacher; they were commitments that students accepted voluntarily for the sake of their own personal health. Naturally, each commitment a student undertook depended on that student’s initial circumstances: absence, deficiency or existence (in the case of bad health habits). The foremost achievement of the activity was the high degree of student involvement, with a total of 128 commitments to good health, an average of 2.46 commitments per student. The second most important achievement was the highly meaningful improvement (p <= 0.001) of habits related with breakfast and fruit and the remarkable evolution (p <= 0.05) of a) intake of water, soft drinks and pastry, b) physical activity and c) dental hygiene and posture. Therefore, the results support the methodology and thus its suitability for promoting healthy living habits and/or physical activity in teenage students.