Young people now more than ever live in a diverse, complicated world. In spite of the qualitative proliferation of differences, however, the current social situation in late-stage capitalism is articulated as superficial multiculturalism. Critically examining how bodies materially experience cultural boundaries is then important for enhancing the discussion on "multicultural co-existence" in Japan. This article explores how Japanese youth experience their senses of self in relation to ethnicity/nationality through looking into drama/theatre classes at a high school. Regarding school as an important site for youth self-formation, this research is situated within the critical educational research body exploring the interplay between schooling and youth affect. Applying Deleuze and Guattari's concepts such as "territorialization" and "the education assemblage," I examine the ways in which young people experience ethnicity/nationality as a singular event. The analysis is based on seventeen months of ethnographic research with high school students in Tokyo. I specifically look at their practices of mimesis, mimicking "the other" in acting or performance in drama class. Utilizing the tools of feminist poststructural ethnography, I analyze the narratives that emerged from observation in drama classes and in-depth interviews along with journal writing and assignments. As seen through repeated patterns, the analysis reveals how the identity categories of ethnicity/nationality rigidly function to territorialize youth, blocking possibilities for them to become otherwise. I then address some views that might have produced these experiences: humanistic views of people, formalistic educational aims and discursive practice around ethnicity/nationality. In conclusion, I refer to points toward oppositional educational practices at school. These points include critical reflection on cultural differences being embedded in educational practices and active engagement with precarious relations. Finally, reframing learning by centring youths' bodies, I call for more educational research with a Deleuzian approach. By shedding light on the complexities of the affective experiences of youth, this study will hopefully contribute to promoting the discussion of democracy at schools in Japan.