Numerous authors in the library and information science (LIS) field have called for more authentic collaborative experiences for students in school librarian education programs, particularly experiences that partner school library students with pre-service teachers to collaboratively design instruction. The first-iteration, design-based study described below examines the impact of such a project, a collaborative science-focused lesson plan assignment given to pre-service elementary school teachers (PSTs) and pre-service school librarians (PSLs) at a public university in the southeastern United States. Specifically, this paper explores the impact of the project on school library students' understanding of collaboration between teachers and school librarians, particularly science-focused collaboration, and examines the features of the assignment that either facilitated or hindered their progress toward improved understanding. Based on the results of this project, we provide recommendations for school librarian educators interested in designing and implementing similar projects.