Context: It is common practice in academia to have students work with "toy" projects in software engineering (SE) courses. One way to make such courses more realistic and reduce the gap between academic courses and industry needs is getting students involved in open source projects (OSP) with faculty supervision. Objective: This study aims to summarize the literature on how OSP have been used to facilitate students' learning of SE. Method: A systematic mapping study was undertaken by identifying, filtering and classifying primary studies using a predefined strategy. Results: 72 papers were selected and classified. The main results were: (a) most studies focused on comprehensive SE courses, although some dealt with specific areas; (b) the most prevalent approach was the traditional project method; (c) studies' general goals were: learning SE concepts and principles by using OSP, learning open source software or both; (d) most studies tried out ideas in regular courses within the curriculum; (e) in general, students had to work with predefined projects; (f) there was a balance between approaches where instructors had either inside control or no control on the activities performed by students; (g) when learning was assessed, software artefacts, reports and presentations were the main instruments used by teachers, while surveys were widely used for students' self-assessment; (h) most studies were published in the last seven years. Conclusions: The resulting map gives an overview of the existing initiatives in this context and shows gaps where further research can be pursued.