Table 1 The Healthy High School (HHS) intervention components

1) The HHS curriculum

The curriculum (accessible in Danish here: www.sdu.dk/da/sif/bgym) is based on behaviour change techniques to change social norms and cognitive factors such as knowledge, awareness, skills, and attitudes. It consists of a total of 17 mandatory lessons (1440 min) for four different subjects (Danish, Social Studies, Physical Education and Sport and Introduction to Natural Science), and optional lessons within a Multi-Subject Coursework (one school week). Activities focusing on healthy eating habits and meal frequency are included in 5 lessons (405 min) and consist of 1) information on national dietary guidelines and training of skills to critically evaluate different sources of dietary advise, 2) sociology of food (Discussion of the importance of norms, habits, and health inequalities on food habits), 3) the interrelations between eating habits and meal frequency and stress, sleep habits, and physical activity. All lessons were designed to cover official learning goals defined by the Danish Ministry of Education and were intended to replace some of the standard lessons for the subjects mentioned above. The teachers were introduced briefly to the material at a pre-intervention kick-off conference but were expected to be able to deliver the activities without prior training.

2) The HHS catalogue – organisational and environmental changes for a supportive school environment

The catalogue (accessible in Danish here: www.sdu.dk/da/sif/bgym) consists of 16 initiatives addressed to school management, canteen staff, student councils, teachers, and student counsellors. Two initiatives aimed directly at creating a school environment that support regular meals, and intake of water and healthy snacks.

Initiative 1: A visit from a professional canteen consultant at the beginning of the school year. The consultant encouraged the canteen staff to adjust their food selection to support regular meal frequency, healthy snacks, and intake of water. The consultant had formal training in nutrition and health and profound experience in consulting canteens. The canteen consultant used the validated scoring system “Kantinetjekket|Buffet” (in English “Canteencheck|Buffet”) [42] to rank the canteens nutritional value. Based on the result, the consultant guided the canteen staff in implementing changes that could improve the nutritional value of their selection of food and nudging strategies which could promote healthy choices. The initiative focused on changing one or a few things at a time and to substitute existing products with a healthier version, e.g., white bread and pasta with wholegrain versions. Halfway through the school year, the canteens received a mini catalogue with further guidance and ideas to make a healthier canteen (accessible in Danish here: www.sdu.dk/da/sif/bgym). This catalogue was based on the canteen consultant’s experiences from the visits at all intervention schools and developed together with the project group. Initiative 2: We encouraged the school management to enable access to cold, clean water from a hygienic source. Three suggestions were given: 1) installing water fountains, 2) make pitchers with cold water available daily at the school area, 3) to provide easily accessible and cheap bottle water.

The catalogue also includes other initiatives to support healthy eating habits and regular meals e.g., formulation of a health- and well-being school policy and common breakfast for all first-year students. The catalogue was presented to representatives of the intervention schools at a pre-intervention kick-off conference.

3) Young & Active – a peer-led innovation workshop and derived activities

Based on the needs assessment the innovation workshop specifically aimed at inspiring students to develop and implement new activities to promote physical activity and sense of community at school and did not address healthy eating habits and meal frequencies. The workshop was based on a peer-led approach, student co-determination, innovation techniques and local solutions. At all intervention schools the workshop was facilitated during school hours by university students in Sport Science and Health (peer mentors) within the first months of the intervention. The workshop included two parts of 90 min each, held at two separate days with one or two weeks in between.

4) The HHS smartphone application (app)

The app aimed to support and promote healthy habits and well-being outside school hours. The app is designed to influence student’s skills, knowledge, awareness, outcome expectations, and attitude. The app includes e.g., 1) healthy breakfast, lunch, and snack recipes, 2) meal tracking options, 3) quizzes, tests, and debunking myths on healthy eating habits and regular meals. Furthermore, the students could sign up for an eight-week text messaging programme on how to e.g., improve eating habits. Contrary to the original implementation plan, the app was introduced to the students 12 weeks after the introduction of the intervention due to a delayed delivery from the app developing company which hindered students’ uptake of the app. The download of the app was promoted by teachers at the school and the project group and needed a code to install. (The app is no longer available for download).