Good teachers vs. bad teachers
The L.A. Times collected data, via information request, on student achievement in the L.A. Unified School District, then handed the information over to the Rand Corporation for study. Using value added assessment, Rand and the L.A. Times found that there was a night-and-day difference between the quality of teachers at many schools. Some teachers were so bad that their students fell behind their peers. Others, so good that their students went from being below average to above average.
Unfortunately, union-imposed rules prevent school districts from rewarding high-quality teachers. Unions prefer the mind-dulling, lock-step pay system that rewards teachers for years of service rather than quality and, as a result, promotes mediocrity.