In this paper, we report on the NSF-funded project Partner4CS which seeks to foster the development of computational thinking in K-12 settings through a strong partnership of university faculty, computer science undergraduates, teachers and students. We describe two afterschool programs focusing on game design, designed by the Partner4CS team in local schools serving a diverse population of students, including those with mild learning and social emotional challenges. We subsequently explore the potential of game design to promote computational thinking at the middle school level as well as its impact on student attitudes and motivation towards computing. Findings indicate that game design helped students acquire computational thinking concepts supported by Scratch as well as an understanding of the design process. Findings related to attitudes towards computing demonstrated mixed results. Implications are drawn for strategies that support computational thinking at the middle school and the role of university partners and computer science undergraduates.