The thesis is focused on prevention of homophobic bullying that can result from homophobic and hetersosexist attitudes. It is based on action research in two peer groups in 8th grade classes. The empiric part included initial mapping of knowledge of students on various sexual and gender identities and their attitudes towards non-heterosexual people via questionnaires; development and implementation of a primary prevention programme on non-heterosexual identities, heteronormativity hompphobia and homophobic bullying; evaluation of the programme via questionnaires; and final evaluation of changes of attitudes of the target group. The theoretical part covers current knowledge on the main aspects of the research toppic (bullying, homophobic bullying at school environment and peer collectives) and the area of primary prevention of negative phonemena. It is based on concepts of homophobia, heterosexism and heteronormativity.