This study is inserted in current investigations about the goals of formal education and about the fulfillment of the promise schools make: social rise to all students. Focusing on our public school system, we investigated the conditions under which quality teaching is produced. This investigation consisted of a case study referring to Humaitá II, one of the thirteen branches of Colégio Pedro II, a teaching institution which is considered a landmark in Brazilian education, as the history of this school is closely related to the very history of the country, in social, political and cultural terms. Also, this school is seen by Brazilian population as a producer of social elites. We started with the hypothesis that students` proficiency is directly influenced by the atmosphere surrounding the school they attend. We focused on some of the aspects related to this atmosphere – the ones that were highlighted as capable of making a difference in terms of quality teaching. The notion of school atmosphere appears as a crucial factor to the study of school identity. As a social organization, each school has its own way of operating and its own social relation system. The field work was conducted in the years of 2007 and 2008 and data was collected in the following ways: observation of classrooms and other collective spaces, documental research and semi-structured interviews, both formal and informal, with teachers, students and other school workers. In order to use the concept of school atmosphere properly and analyze the data, we took the works of Antonio Nóvoa, Leila Mafra, Luc Brunet, Olivier Cousin, Pascal Bressoux, Teixeira Lopes and Manuel Jacinto Sarmento our theoretical and methodological reference. Through the analysis of the empirical data it was possible to identify the singular configuration that the Humaitá branch has, that is, the set of characteristics that account for its public image of excellence. These characteristics are: the identification of the members of the school with the identitary universe of Colégio Pedro II, the feeling of belonging to this universe, the variety of social origins of its students and the qualification and commitment of its teachers. All of these constitute the institutional singularity of the Humaitá branch and make it possible for Colégio Pedro II, the biggest Brazilian public school of elementary and secondary levels, to provide quality teaching and socially valued school results.