The aim of this paper is to contribute to the development of a research agenda on organizational school practices as well as the organizational knowledge derived from such practices. In order to meet this goal, the main findings of a research project about the organizational characteristics of schools that had developed sustainable improvement processes over time will be described. The project followed an ethnographic methodology based on indepth case studies of five primary schools, four secondary schools and one special education school in the provinces of Sevilla and Gran Canaria, both in Spain. First of all, the paper will briefly frame the idea of professional communities of practice. After that, three cases will be synthesized to exemplify different trajectories for improvement. While two of them can be considered successful trajectories, the third appeared to be a relative failure. The contrast between these cases will let to us to conclude and discuss about the organizational characteristics that take part in educational change processes. The main of such conclusions is that the strongest basis for school improvement is the building of professional communities of practice at the organizational level. These communities take advantage of the difficult circumstances of the school to create a strong collective identity among teachers and also a strong feeling of institutional affiliation. For that purpose they rely on the storytelling activity which gives sense to the improvement effort as well as lending a missionary feeling to teaching. The configuration of such care cultures, able to sustain improvement efforts over time, is completed by both, active socialization processes of the new members and widely distributed patterns of leadership.