Purpose: Due to the decline in school-age populations and urbanization, the discussion on integrated elementary and middle schools has begun in earnest. To effectively operate integrated schools, it is necessary to refer to the experiences of countries that have long implemented similar systems. Thus, this study aimed to explore the comprehensive school system in Northern Europe, where primary and lower-secondary schools are integrated, and analyze its characteristics to draw implications for the operation of integrated elementary and middle schools in the South Korean context. Methods: To achieve the research objectives, this study examined and analyzed the background and current status of the comprehensive school systems in Northern European countries—Finland, Norway, Sweden, and Estonia—that operate under a single structure integrating the primary and lower-secondary school education periods. The study also explored the teacher training systems, school evaluations, and student assessments in these countries. Results: The four countries aimed to achieve both social welfare for all citizens and educational effectiveness. The comprehensive school systems in each country share a common feature of striving to realize the values of social democracy. To link the two school levels, each country has developed its own teacher training and evaluation systems. Conclusion: These findings suggest that, in order to effectively operate integrated elementary and middle schools in the Korean context, it is essential to develop methods that link and integrate the two school levels while considering the specific characteristics of each level, promote collaboration among teachers, and clearly establish educational objectives in addition to administrative and economic goals.