Starting early in life, children, especially girls, experience obstacles when it comes to developing interest in STEM. Although teachers face an important task in promoting girls (and boys) in STEM, they often encounter hurdles in doing so. A three-month-long training for pre- and in-service teachers in elementary education was developed to counter this phenomenon. An important training feature was teaching ideas for STEM classrooms. Teachers’ evaluation of the training and teaching ideas, changes in their self-concept, and elementary students’ assessment of the teaching ideas were investigated. Students rated the teaching ideas favorably, with no gender differences. Even though each idea incorporated relevant didactic features for gender-sensitive STEM instruction, the teachers rated certain ideas and contents more critically than others. Nevertheless, their assessments speak in favor of the training intervention, while also indicating gaps in teachers’ professional knowledge regarding gender-sensitive didactics. Implications for the design of STEM teacher education are outlined.