Background and Context: A key challenge in advancing computer science education in K-12 schools is teacher preparation and support. School-university partnerships and service-learning programs where undergraduates assist teachers represent one promising approach to supporting K-12 computer science teaching. Objectives: In this work, we examine the computer science content and pedagogical practices that undergraduates knowledgeable in computing reported implementing in order to support K-12 teachers in their classrooms as part of a university service-learning course. Method: Data were collected from 60 undergraduates enrolled in the course over nine semesters. Data included weekly reflective entries, course observations, observations in K-12 classrooms, and collection of artifacts. Data were analyzed using qualitative techniques and descriptive statistics to identify dimensions of high-quality computer science content and pedagogy reported by undergraduates. Findings: Findings indicated that undergraduates were able to connect knowledge of computing to pedagogy and technology to assist teachers in the implementation of computer science instruction. Specifically, undergraduates worked with partner teachers to employ high quality content and practices that made computer science accessible to students. Implications: Findings from this work are important as different models are considered for how to provide professional development and/or support K-12 teachers in building knowledge needed for sustainable computer science teaching.