This paper aims to explain the underground curriculum in the field of ethnicity and provide its practical evidence from the Iranian schools. This paper follows a qualitative research method and the tools and methods for collecting data include interviews and observations. According to the results of this research, when ethnic groups do not have a proper position in formal curricula, and in many cases, there is a cultural humiliation of subcultures; in other words, when cultural occupation takes place in schools, in such circumstances Ethnicity is considered as one of the main areas for the formation of underground curricula. Also, based on the data analysis, seven main themes related to this research are: preference for mother tongue on formal language, ethnicity bias of teachers and students, opposition to the content of formal curriculum, the use of ethnic symbols in schools, the spread of ethnic music and their symbols in schools, humiliating other ethnic groups and expansion of separatist ideas. Also, based on this research, the formation of underground curriculum in the field of ethnicity in schools can be explained based on resistance theory.