Schools provide an important setting for health promotion and health education. In countries where health education is not a specific subject, it is typically undertaken by teachers in health-integrating subjects such as biology, home economics or physical education. More ambitious and holistic frameworks and whole school approaches such as health promoting schools have been considered best practice for the past three decades. Recently, more attention has been given to policy initiatives integrating health activities into school curriculum time. This paper discusses potentials and challenges of school-based health promotion applying an 'add-in' approach, that integrates health activities into teachers' curricular obligations without taking time away from them, based on a presentation of three Danish cases. This may serve as a supplement to health promotion activities that have been initiated over and above the day-to-day teaching (add-on). We contend that an 'add-in' approach to school health promotion provides a potential win-win situation where both health and core education stand to gain; makes it possible to reach a wider range of schools; mobilizes additional resources for health promotion; and leads to more sustainable activities. However, potential limitations including not addressing structural aspects of health promotion and reliance on a relatively limited evidence base should also be considered.