In order to understand how the gap between the goals and the results of initially intended educational policies arises, this study adopted the concept of policy slippage and applied it to the case of policy on autonomous public high schools. It was the participatory government that started implementation of the policy of open autonomous high schools. And the policy was succeeded by the Lee Myung-bak government, while the name of the high schools was changed to autonomous high schools. However, when the Park Geun-hye government took over the policy, it was considered as a failure. The results of the analysis in this study indicate that there occurred continuous and cumulative slippage of policies due to political interactions such as power struggles, rejection, mediation, and support among the Ministry of Education, the ruling party, the opposition party, educational organizations, civic groups, local governments, and the principals of autonomous high schools. There arose phenomena such as temporal slippage where the implementation of determined policies are delayed and content slippage where the initial intention and contents of the given policies are modified. These phenomena resulted from power imbalance among policy agents. That is, when the Ministry of Education exerts an excessive influence over the policy decisions or conversely, when stakeholder groups and political parties exert strong impact on them, the given policies were distorted or went awry. At this end of this study, it was suggested that a structural device which secures political rationality be established in order to minimize the loss resulting from policy slippage.