Effective schools should be superior in both enhancing students' achievement levels and reducing the gap between high- and low-achieving students in the school. However, the focus has been placed mainly on schools' achievement levels in most school effect studies. In this article, we attend to the school-specific achievement dispersion as well as achievement level in determining effective schools. The achievement dispersion in a particular school can be captured by within-school variance in achievement (sigma[superscript 2]). Assuming heterogeneous within-school variance across schools in hierarchical modeling, we identified school factors related to high achievement level and a small gap between high- and low-achieving students. Schools with a high achievement level tended to be more homogeneous in achievement dispersion, but even among schools with the same achievement level, schools varied in their achievement dispersion, depending on classroom practices. Estimated mean and 95% document interval for 143 schools is appended. (Contains 4 tables and 4 figures.)