A promising approach to make K-12 Computer Science education more systemic could arise from a strategy focusing mostly on pre-service teachers educated through mandatory courses instead of self-selected in-service teachers. When employing mandatory courses, schools of education can reach all future teachers, but what are potential consequences resulting from this demographic shift? Pre-service teachers may not expect to acquire programming skills and may not be convinced of the relevance of Computer Science. In 2017, one of the first mandatory Computer Science education courses for pre-service K-12 teachers was introduced at the School of Education of northwestern Switzerland (PH FHNW). The mandatory nature of the course was possible because of the introduction of Computer Science as a subject in a new national curriculum. The course, based on Scalable Game Design, was taken by over 600, mostly female (75%), pre-service elementary school teachers. This paper explores the characteristics of this new audience and investigates the consequences of mandatory pre-service teacher Computer Science education. While our research shows that the course was successful, with regards to improving the students' skills, it reveals significant gender effects concerning attitudes towards Computer Science and self-efficacy.