This article describes lessons learned about grouping by teachers and teacher educators who worked collaboratively to design and deliver a summer writing institute for a heterogeneous population of students about to begin ninth grade at an urban high school. Institute staff used co-teaching (Cook & Friend, 1995) extensively to provide support for student learners as well as to encourage professional learning about teaching writing. Such co-teaching allowed teachers to model expectations for academic tasks and behavior explicitly, create student groups that drew on multiple perspectives, approach teacher-student writing conferences deliberately, and provide extra scaffolding to youth in need of it. The article concludes with suggestions for applying the author team's insights about grouping and co-teaching to more traditionally organized literacy instruction during the academic year. (Contains 1 figure.)