本文探討十二年國教免試入學所帶來的公平與學生成就之間的取捨關係。本文採用二重差分法（DID），比較受政策影響的高中生與未受影響的國中生。結果顯示，改革使高中校內學業表現異質性增加 30 分（效果量 0.86），顯示以能力為基礎的校際分流顯著弱化。同時，低成就學生進入排名較高學校、接觸較優質教師及高成就同儕的機會明顯改善。然而，這些公平收益伴隨顯著學業代價：整體學業表現下降 50 分（0.18 個標準差），且負面影響均勻分布於整個成就分配。整體而言，研究顯示免試入學政策在擴大弱勢學生教育機會的同時，可能犧牲整體學業表現。

This paper examines the trade-off between educational equity and student achievement by analyzing Taiwan’s 2014 exam-free high school admission reform, which substantially reduced ability-based tracking. Using a difference-in-differences design comparing high school students exposed to the reform with unaffected middle school students, this paper finds that the reform increased within-school heterogeneity by 30 points (effect size=0.86), evidence of a major weakening of ability-based sorting across schools. Consistent with the policy’s equity objectives, low-performing students gained access to better schools, more qualified teachers, and higher-achieving peers. These equity gains, however, came at a clear cost: average performance declined by 50 points (0.18 standard deviations), with negative effects spread uniformly across the achievement distribution. A triple-difference design analysis yields consistent results. Overall, this study’s findings reveal a pronounced equity-achievement trade-off, showing that detracking can expand educational opportunity while reducing overall academic performance.