A great deal of knowledge has been accumulated on how educational effectiveness and improvement come to be. However, the issue of how these processes behave once they have been achieved has remained relatively unexplored. In this paper, we report on the findings of a multiple case study that delved into the factors and processes explaining different levels of sustainability of educational effectiveness for schools previously identified and studied as highly effective and located in low-income areas. We show that effectiveness is a fragile attainment that is affected by a variety of external and internal processes. Transformations in the local context (demographic changes and the dynamics of the school market), and educational policies and programs are key external factors. In the internal realm, the role of school administrators, principals, and leadership teams, the organization of pedagogical work, and the school culture modulate the sustainability of educational effectiveness in the long term.