This research aimed to analyze school principals’ practice in Brazilian public schools. We sought to examine it from three sociological perspectives: school principals as workers (analyzing their actual work), school principals as public officials (analyzing their action in the implementation of an educational public policy), and school principals as educational leaders (analyzing the development of their leadership at school). For this purpose, fieldwork was conducted in four full-time schools (PEI Program) of the São Paulo State education system in Franca, Brazil. The methodology combines semi-directed interviews with four school principals and five deputy principals; and the unstructured observation of one working day of four school principals. The analysis shows that school principals’ work routine consists of several unpredicted demands and interruptions, with significant relational work. Moreover, in the case of the PEI Program, the principals showed a great alignment with the policy, acting as true advocates of it locally. Finally, we observed both managerial and leadership approaches in their practice. It was also possible to notice a change and update of the function over time.