Diverse initiatives have emerged to popularize the teaching of computing in K-12 mainly through programming. This, however, may not cover other important core computing competencies, such as Software Engineering (SE). Thus, in order to obtain an overview of the state of the art and practice of teaching SE competences in K-12, we carried out a systematic mapping study. We identified 17 instructional units mostly adopting the waterfall model or agile methodologies focusing on the main phases of the software process. However, there seems to be a lack of details hindering large-scope adoption of these instructional units. Many articles also do not report how the units have been developed and/or evaluated. However, results demonstrating both the viability and the positive contribution of initiating SE education already in K-12, indicate a need for further research in order to improve computing education in schools contributing to the popularization of SE competencies.