The main aim of this research was to investigate the knowledge and attitudes of high school teachers about the hidden curriculum with special emphasis on values in the teaching environment, the self-assessment of teachers about the importance of their role in shaping the value system of their students as well as the elements of the hidden curriculum that teachers implement in the teaching process. For the purpose of the research, a semi-structured interview was used in which a total of seven high school teachers were interviewed: five from the Republic of Croatia and two from Texas. The research has shown that the concept of hidden curriculum is not well known among the examined high school teachers, although they implement it in their classrooms. Namely, the emergence of this pedagogical phenomenon is an indispensable part of every classroom environment. Despite the fact that teachers are not sufficiently familiar with the terminology of the hidden curriculum, they introduce elements of the hidden curriculum into the educational process. Teachers' core life values, educational values, and the rules they bring to the classroom environment are elements of the hidden curriculum that are omnipresent in the twenty-first century classrooms.