Using a nationally representative sample of eleventh grade students in South Korea, we investigated how the residentially based school assignment policy called the High School Equalization Policy (HSEP) shaped the separation of low and high socioeconomic status (SES) students between schools. We found that there was a smaller between-school separation in the HSEP regions than in the non-HSEP regions. We also found that student achievement significantly depended on a school's mean SES only in the non-HSEP regions. We discuss the implications of these findings for the potential impact of school choice policies on educational inequality for Korea and beyond. (Contains 4 tables and 12 notes.)