This article contrasts policy intent and policy implementation in school autonomy and accountability. The analysis uses a conceptual framework based on the interaction between school autonomy, student assessment, and accountability as elements of a closed system. The article analyzes the implementation of school autonomy and accountability policy, using data collected from 226 schools in Thailand that participated in the 2009 PISA survey. To understand how autonomy and accountability relate to student achievement, the study estimates a linear regression with PISA reading achievement as the dependent variable and indicators of autonomy and accountability (and control variables) as the independent variables. The estimates suggest that students at schools exercising a higher level of operational autonomy than the level ascribed by regulation tend to have PISA reading scores 6.0-8.6 points higher than students at schools that behave less autonomously. These results are consistent with other research findings.