Executive coaching has become increasingly commonplace in both the corporate and non-profit sectors as a means of improving professional effectiveness but there is a dearth of empirically-based protocols geared specifically toward the growth needs of school principals. This qualitative case study explores the implementation of a principal coaching protocol using a "theories of practice" framework based on concepts originally articulated by Argyris and Schön (1974) and further explicated by the authors in previous publications. This study examined the extent to which a coaching protocol based on theories of practice enhanced principals' self-perceived capacity for reflection and effective instructional leadership. Findings suggest that principals valued the structure, feedback, and reflective dimensions of the protocol and found their confidence level about an important instructional leadership problem--how to support and assist struggling teachers improve their teaching practice--was greatly enhanced. Implications for further iterations of the coaching protocol, as well as future directions of research on principal professional growth, are discussed.