This study casually investigated the effects of co-education in middle school on recognition
of school climate and study time. This study may contribute to providing information for the
way of enhancing the role of co-ed school practicing gender equity. This study hypothesized
that co-ed school will influence students' behavior and academic achievement from the
distraction perspective in co-ed school. Theoretically, dealing with endogeneity is a key issue in
studying of co-ed school effects. This study used instrumental variable indicating whether
students reside in the area having both co-ed school and single-sex school. The presence of
single-sex school in residence will influence externally the probability of being allocated to
co-ed school. This study showed that instrumental variable significantly accounted for the
entrance of co-ed school. When the whole sample was investigated, the endogenity of co-ed
school was denied in the effects on school climate and study time. However, when sample was
divided into subgroups by sex and family background, the endogenity of co-ed school was
effective to boys with lower social economic background. This means that results from OLS for
co-ed school effects will be biased when sample was constrained to these boys. This study
demonstrated that co-ed school exerts strong-negative effects on school climate and study time
to poor boys. Results indicate that to increase co-ed school, negative effects toward these boys
should be appropriately dealt with.