This study describes classroom code-switching practices in South Korean high schools after the South Korean Ministry of Education requested that English teachers maximize their English use. The data comprised the recorded language from 13 high school English teachers' classrooms and teachers' and students' responses to surveys asking about their reactions to the call for maximal use of English in class and the challenges they are facing. The data analysis indicates that (a) the teachers used on average a rather low amount of English (32%), lower than what they and their students considered appropriate (53%–58%); (b) the teachers' code switching followed certain patterns and principles, although it was often not principle governed, and their use of Korean (L1) was very effective for several functions; (c) teachers' beliefs tended to affect their code-switching practices; (d) teachers' language use appeared to affect students' language behavior in class, although students' decisions on what language to use often depended on the question's complexity and level of difficulty; and (e) curriculum guidelines seemed to affect teachers' language use, but factors like teachers' beliefs and teaching contexts might severely mitigate their impact. Pedagogical implications and suggestions for further research are also discussed.