Using data from the Programme for International Student Assessment (PISA) and Trends in International Mathematics and Science Study (TIMSS), this study documented how academic achievement of elementary, middle, and high school students from middle-class families had changed over time from 1995 to 2019 in the United States and South Korea. In addition, this study examined how the achievement gap between lower- and middle-class students and between middle-and upper-class students changed during this period time. The overall mathematics and science achievement of American elementary school students from middle-class families (defined by those at the 50 percentile of the socioeconomic status distribution) increased from 1995 to 2011, but it had declined since then. By contrast, both mathematics and science achievements of American middle school students from middle-class families had increased from 1995 to 2019. However, the overall reading and mathematics achievement of American middle-class students had slightly declined from 2000 to 2018, while their science achievement had slightly increased. For South Korea, both mathematics and science achievements of middle school students from middle-class families had increased from 1995 to 2019. By contrast, the overall performance on reading, mathematics, and science among Korean middle-class high school students had declined from 2000 to 2018. When it came to the achievement gap across different social classes, the achievement gap between lower-class students (defined by those at the 10 percentile of the socioeconomic status distribution) and middle-class students had declined over time from 1995 to 2019 at the elementary school level, while that between middle-and upper-class (defined by those at the 90 percentile of the socioeconomic status distribution) students had increased in the United States. The achievement gap between these social groups of students had little changed from 1995 to 2019 at the middle school level in the United States, while they had declined from 2000 to 2018 at the high school level. For South Korea, the achievement gap between lower- and middle-class students and between middle- and upper-class students had slightly declined over time from 1995 to 2019 at the middle school level, while it had increased over time from 2000 to 2018 at the high school level, especially for reading achievement.

Soo-yong Byun is a professor of Education, Demography, and Asian Studies at the Pennsylvania State University. He is also a core faculty member and Professor-In-Charge of Penn State's Comparative and International Education Program.

He was born and bred in a small rural town in South Korea and earned his BA in Education and MA in Sociology of Education in Korea University. He earned his Ph.D. in Educational Policy and Administration with Comparative and International Development Education track from the University of Minnesota at Twin Cities. Prior to joining the Penn State faculty in 2012, he worked as a postdoctoral scholar at Michigan State University (2007-2008) and the University of North Carolina at Chapel Hill (2008-2011).

His scholarly interests include sociology of education, international comparative education, rural education, and educational policy analysis and program evaluation. The articles of his research have been published in leading peer-reviewed journals, including American Educational Research Journal, American Journal of Education, Comparative Education Review, and Sociology of Education.

He served as the president of the Korean American Educational Researchers Association (KAERA) for 2017-18 and as a visiting scholar at Seoul National University's Department of Education for 2018-19 . He will serve as a non-residential visiting professor at Ewha Womans University's Department of Education for 2022-23.