High schools have traditionally worked on improvement in isolation, but a growing number have broken the barriers of isolation by working with a school improvement coach. An outside school coach, properly prepared and sensitive to individual and whole-school concerns, can provide a balance of pressure and support to initiate and sustain meaningful school improvement. Unlike a consultant, a coach has a long-term agreement with a school. Their sole responsibility to a school is to support its improvement, very different than just adding improvement to the already growing list of tasks of school leaders. With colleagues at the Southern Maine Partnership, the authors have developed a school coaching model refined and revised over 20 years, in which it has supported improvement in dozens of high schools in the region. Its five core beliefs which seem familiar but are too often overlooked or underestimated are discussed in this article. Shared leadership is essential. School improvement doesn't happen on its own and takes time to do right. It cannot be the purview of just one person and is not a top-down endeavor. A leadership team composed of the principal and several teachers charged with shepherding the school improvement work is necessary. Although a consultant often plays instigator or catalyst for change, a coach goes a step further by establishing ongoing working relationships with members of the school community to see the change through. Just as every student needs customized support to be successful, every high school needs similar support to meet its school improvement imperative. School coaching is one way to meet that need.