In my work in schools over the last few decades, three hard truths have gradually come into focus. First, within every school there is a bell-shaped curve of teaching quality. Some teachers are using highly effective practices almost all the time, a larger group is solidly effective, others are working at a mediocre level, and a few are mostly unsuccessful with their students. Although the curve may tilt left or right from year to year and place to place, teaching variability is a reality everywhere, from struggling inner-city schools to expensive prep schools. The second hard truth is that teaching practices at the lower end of the spectrum have a disproportionately negative impact on some students. Children who are fortunate enough to enter school with