Background: Adequate dietary habits and physical activity during childhood and adolescence may promote growth and cognitive development and contribute to the prevention of chronic disease in later life. School is considered an important social environment that can promote healthy eating habits and life-style changes.

Objectives: To evaluate the effects of a school-based intervention on nutritional knowledge, eating habits, and physical activity of adolescents.

Methods: We conducted a prospective questionnaire-based study. Anonymous questionnaires were administered at the beginning of the academic year (September 2014) in one high school. During the following year, vending machines containing milk products were installed within the school facility, and students were given two informative nutrition lectures regarding proper nutrition for age, calcium requirement and importance, and physical activity. One active sports day was initiated. At the beginning of the following academic year (September 2015), the students completed the same questionnaires.

Results: The study was comprised of 330 teenagers, mean age 15.1 ± 1.39 years, 53% males. Response rate was 83.6% ± 0.4% to multiple choice questions, 60.7% ± 0.5% to multiple section tables, and 80.3% ± 0.9% to open questions. Post-intervention, respondents reported an increase in eating breakfast (57% vs. 47.5%, P = 0.02) and a decrease in purchasing food at school (61.6% vs. 54.3%, P = 0.03). No changes were observed in consumption of milk products, knowledge regarding calcium and vegetable consumption, or sports activities.

Conclusions: Short-term high school-based interventions may lead to improvements in eating habits but are not sufficient for changing nutritional knowledge and physical activity