The current efforts to expand computer science (CS) education in K-12 schools, such as the "CS for All" initiative, highlight the need for all students to get an opportunity to study computing. However, as recent research has shown, diversity in computing at the K-12 level remains problematic, and additional research is needed to look at how computer science learning environments can impact minority student interest and retention in CS. In this article, we report results from an in-depth qualitative study of high school computer science teachers' perspective on barriers to increasing diversity in their classes. Based on teachers' experiences, we provide practical recommendations on how to encourage equitable learning environments in K-12 computer science courses.