This study investigated the factors of increased English instructional time at schools along with
the causal relationship between this increase in English instructional time and achievement scores in
the academic-oriented high schools in Seoul. We employed the instrumental variable (IV) methods
in order to verify the effect of increased English instructional time school policy. We noted that the
decision-making of a curriculum at schools is a process of conflicts and negotiations among the
main actors of a school, such as parents and teachers, even though the authority of
decision-making for a curriculum is endowed to the individual school from the national government.
As for the traits of schools that have more English instructional time than other subjects, we found
significant relationships between English instructional time and the principal\'s perception on the
venue of a curriculum decision, the implementation of subject classroom system, and schools that
were designated as autonomies by other government policies. Using these three variables showing
significant relationships with English instructional time, we demonstrated that the decision on
increasing English instructional time was made by the endogenous process and that the three
instrumental variables satisfied the conditions of relevance and validity. Further, this study showed
a causal relationship between increasing English instructional time and achievement scores, which
existed when employing those instrumental variables. In addition, the effect of increasing English
instructional time was found only in students who were below the 8th grade. We discussed
implications of these results in respect to how and why schools should increase their English
instructional time. Lastly, we discussed the establishment of school governance in reducing conflicts
among the actors when school has the autonomy over deciding a curriculum.