This article examines the use of interdisciplinary project co-design, as a mechanism for increasing the capacity of a school and promoting the growth of teachers' professional practice in an urban high school setting. Changing teaching practices and the professional culture within a school can be extremely difficult. Simply providing resources about novel strategies can be ineffective. In fact, in some school cultures, suggestions for classroom practice change can be received with hostility, being viewed by some teachers as acts questioning their professional competence. This study describes how a strategically chosen task, interdisciplinary project co-design, was used by external technical assistance providers as a productive, non-threatening mechanism for instructional improvement, by simultaneously enhancing classroom practices and cultivating the growth of teacher collaborative practices.