The main objective of this study is to analyze the construction of professional identity by teachers at Colégio Pedro II, the leading public secondary school institution in Brazil. Founded in 1837, Colégio Pedro II preserved until the mid-20th century the status and character of model institution at this educational stage in Brazil, a position it still retains by virtue of the federal government. The time frame chosen is the period that extends from 1925 to 1945, delimitated by three reforms (Rocha Vaz in 1925, Francisco Campos in 1932, and Gustavo Capanema in 1942) which consolidated the institutionalization process of Brazilian secondary schooling. This study s main theoretical references derive from the French sociologists Claude Dubar and François Dubet, drawing particularly from the idea of Professional Identity presented by Dubar and Nóvoa (1995), linked to what Dubet has called Institutional Program. The methodology used was the documental analysis of different sources from the period in question, especially the ones found in the collection of the Nucleus of Documentation and Memory of The Colégio Pedro II, in the Center for Research and Documentation of Contemporary Brazilian History at the Getúlio Vargas Foundation (CPDOC/FGV), as well as in the legislation concerning the educational system in the period. Following these sources, it was possible to identify virtually all teachers who worked in the institution at that time, and the several categories in which they fell; to observe that among those who were placed in the lowest hierarchical categories in the school, some were able to become professors and principals in the years or decades that followed; to discern the teachers attributions; the behavior inside classrooms; the collective action; the contests; some political infighting; salaries; several personal characteristics and other remarkable aspects in the construction of their career paths. The data collected also includes the training of the great majority of teachers, which disciplines they have taught, other institutions in which they worked, whether they were considered good, inexperienced or young for teaching, as well as who owned the Registration, established by Campos reform in 1932. It was even possible to verify how the Colégio Pedro II adapted itself to the reforms imposed by the government and how the teachers reacted to a few of them. Still, we found documents that allow for a distinctive perspective of Colégio Pedro II, revealing a chaotic character that goes against the notions of order, excellence, and standardization, historically built by its teachers, students and employees.