Research highlights the ongoing marginalization, discrimination and bullying directed to minority students. The prevention of such negative behavior towards diversities is an objective of inclusive education. Teachers play a key role in implementing inclusivity by fostering a safe learning environment that promotes respect and protection for every student regardless of their differences. This study aimed to investigate high school teachers’ attitudes, their perceptions towards sexual minority students. The study also examined how competent teachers feel to adapt inclusive strategies in their classroom. A systematic literature review of 33 articles found that although high school teachers’ perceptions and attitudes towards homosexuality, homophobic bullying and integration of related issues in the classroom can be both negative (i.e., exclusive) and positive (i.e., inclusive), their attitudes towards their sexual minority students themselves are mostly positive. Teachers’ lack of inclusivity was mostly attributed to heteronormativity, personal values, fear of losing their jobs, lack of support from administrators, parents and other staff members, fear of disapproval from them, and lack of training. The review also revealed that teachers’ perceived competence towards inclusivity depends on their training, age, years of experience, gender, religion, school type and school region as well as their own sexual orientation. The findings are discussed in terms of the necessity of support from and collaboration with school staff and parents. Furthermore, recommendations are made for school policies that involve sexual minority students, adjustment and for the flexibility of curricula. Lastly, the importance of adequate and effective in-service and pre-service teacher preparation is emphasized.