This study examined the effects of school sectors on student academic achievement and
addressed the school factors that affected these effects. This study made use of the open
enrollment policy for general high school that was implemented in 2010, in which the Seoul
educational authority assigned 40% of students randomly to schools irrespective of students'
preference. Analysis showed that selected students were not different in personal characteristics,
family background and achievement level in middle school by school sectors and thus
confirmed that this study was equipped with characteristics of social experiment. Results show
that private high school enhances students' achievement in English compared to public high
school, but not in Mathematics. Then, the locus of private school's advantage was investigated
using students' evaluation, which could reflect private school's excellence over public school.
Comparison between sectors revealed that students in private school give their schools higher
credits in four fields: enhancement of students learning capability, education appropriate to
students' ability, teachers' commitment, and students counseling. After examining subfactors for
these four fields, this study found that the extent of teachers' informal discussion about teaching
and feedback among peer teachers were highly related to private schools’ effects in
achievement. Implications of this finding were discussed using the concept of learning
organization. This study contributes to literature by confirming the causal relationship between
school sectors and students' achievement but also by addressing the cause of private school's
advantage over public school.