This work aims to provide an overview of Osijek’s secondary education between 1918 and 1941. Moreover, the goal is to determine whether secondary education in Osijek developed or stagnated in the thirty years concerning the number of educated students, the quality of the teaching staff, and the equipment of schools. Using the example of Osijek, the author tries to determine the main features of school policy and teaching at that time, school authorities’ attitude towards teachers and students, the position of students and teachers, as well as characteristics of an average teacher. Classical Gymnasium, Junior Real Gymnasium, Real Gymnasium, and Women’s Real Gymnasium are analyzed. The research methodology is mainly based on the analysis of the educational discourse, the quantitative analysis of sources, and the method of prosopography whose main purpose is to create a collective biography of Osijek’s high school teachers. It is difficult to give an unequivocal assessment of the development of secondary education in Osijek in the period from 1918 to 1941, considering that in some respects it progressed, in others it stagnated, and in others it regressed. The decline of secondary education in Osijek can be linked to the fact that there were three secondary schools in Osijek at the beginning of the observed period, and two at the end of it. Secondary education in Osijek, but also at the level of the entire country, stagnated in most aspects important for the evaluation of the secondary school system. One of the key components for the development of a certain system, including this one, is a financial investment. In the case of secondary schools in Osijek, over 90% of the invested funds went to teachers’ salaries and fees. By simple math, one can conclude that less than 10% of the funds could be invested in the development of the schools, which in no way enabled progress. In that less than 10%, maintenance of school buildings, purchase of consumables and office materials, investment in school libraries, investment in teaching aids, and all other investments had to be included. If all the above is taken into account, it is not surprising that the condition of the school buildings was bad. The situation was similar with the equipment of school libraries and collections of teaching aids. The aspect in which secondary education in Osijek progressed was the increase in the number of students. Between 1918 and 1941, the number of students attending secondary schools almost doubled. In addition to the fact that the number of secondary school students rose during the interwar period, there were also certain changes in the gender, national, religious, and social structure of students. Concern for students’ health constituted an important part of the educational policy. Constant educational and health actions such as teaching students how to better protect themselves, primarily from infectious diseases, but also against venereal diseases and alcoholism represented positive trends in this regard. It was also important to ensure adequate conditions for students’ housing, as well as quality nutrition and the possibility of maintaining hygiene daily. Entertainment was also an important component of the life of high school students in Osijek. As in all authoritarian societies, the state tried to control and manage the free time of young people, that is, to put it within the desired framework. In other words, fun activities were meant to be used for the education of obedient citizens who accept the proclaimed values of the regime. On the one hand, this was tried to be achieved through punishments and the prescription of very strict disciplinary rules. New disciplinary rules for students were prescribed every now and then, and each one was more and more restrictive. On the other hand, efforts were made to control young people through the development of acceptable forms of entertainment. Based on the collective biography of Osijek’s secondary school teachers from the interwar period, an image of an average professor is created. He was most likely born in Slavonia, he was a Croat and a Roman Catholic. After finishing high school, he completed his studies at the University of Zagreb at the age of 24 and a half. He spoke German and French and maybe Italian or Latin. After a year and a half without a job, he got employed in a small community, after which, at the age of 31, he got a job in one of Osijek’s high schools. Then he got married and had one child. Teaching in the interwar period did not differ too much from teaching in the Austro-Hungarian period. Herbartianism still prevailed. The ideas of reform pedagogy and the new school more affected the elementary school system, while secondary schools remained primarily in discussions about curricula. The data obtained from the analysis of secondary education in Osijek can be applied to the entire secondary school system and certain conclusions about the basic settings of educational policy in the first Yugoslav state can be made according to that. Instead of investing in the modernization of the secondary school system and creating optimal conditions for school work, the educational authorities were much more interested in using secondary schools as training grounds for instilling the proclaimed state ideology in future educated personnel. This ideology can be reduced to two main ideas - national unity, that is, especially after 1929, integral Yugoslav identity, and - the leading role of the Karađorđević dynasty. In the future, it will be necessary to carry out more research on the secondary school system in interwar Croatia. Even though this work also deals with the secondary school system as a whole, it is primarily focused on secondary schools in Osijek. Nevertheless, it can be an incentive for conducting similar research in other Croatian cities as well.