Many interest studies have shown the decline of students' interest in physics during secondary education, particularly among girls. Research into physics-related interests of students suggests applying different measures to reduce or reverse that trend such as: (a) suggesting curricular changes that do justice to the specific interests and experiences of girls, (b) improving the ability of teachers to support girls in the development of a positive physics related self-concept, and (c) changing to an organizational setting that gives girls a better chance to improve their self-concept about physics. The purpose of this study was to examine whether these hypothetically effective measures lead to an improvement of the situation for girls when implemented in the physics classroom. The intervention took a whole school year of some 60 one-hour lessons and comprised 12 experimental and 7 control classes of seventh graders (age about 13). Their immediate and long-term achievements, as well as their change of interest in physics, their subjectively experienced competence, and their physics-related self-concept were assessed by written tests at various stages of the intervention. The intervention proved successful and significantly improved most of these indicators for girls (and boys) in the experimental group.