This paper describes an engineering outreach programme designed to increase the interest of under-represented youth in engineering and to disseminate pre-engineering design challenge materials to K-12 educators and volunteers. Given university students' critical role as facilitators of the outreach programme, researchers conducted a two-year design experiment to examine the programme's effectiveness at preparing university students to lead pre-engineering activities. Pre- and post-surveys incorporated items from the Student Engagement sub-scale of the Teacher Sense of Efficacy Scale. Surveys were analysed using paired-samples t-test. Interview and open-ended survey data were analysed using discourse analysis and the constant comparative method. As a result of participation in the programme, university students reported a gain in efficacy to lead pre-engineering activities. The paper discusses programme features that supported efficacy gains and concludes with a set of design principles for developing learning environments that effectively prepare university students to facilitate pre-engineering outreach programmes.