The high school years represent a critical transitional phase during which students navigate career exploration and career decision-making, significantly shaping their lifelong personal development. Policy authorities in Korea have taken proactive measures to strengthen career education within high schools. However, the extent to which students benefit from participating in career education programs remains a subject of debate. Despite increasing interest in career education since the 2000s, empirical studies investigating the causal effects of these programs have been relatively scarce in Korea. The primary aim of this study is to causally estimate the effectiveness of career education programs offered by high schools, and examine how the impact of these activities varies based on the characteristics of participating students, particularly their career plans after high school graduation. To achieve this objective, DID and DDD models were employed, utilizing a balanced panel dataset spanning two years, from the ‘Korean Education & Employment Panel II’. The findings of this study revealed the significant positive impact of career education on high school students’ career development competencies. Notably, career clubs and career experience programs were identified as effective in enhancing students’ career development competencies. Furthermore, the effects of career club and job simulation varied depending on the students’ post-graduation plans. These results underscore the importance of optimizing the educational effectiveness of career education by emphasizing students’ firsthand experiences and considering individual students’ career plans when implementing career education programs.