You seem to be suggesting that the quality of the teacher can play only a minor role in the educational outcome for disadvantaged students. Yet there is considerable evidence to support the opposite view. Here is a summary of a few studies as reported in Teaching as Leadership [1]:
"The schools that are highly effective produce results that almost entirely overcome the effects of student background" [2]
"Having a top-quartile teacher rather than a bottom-quartile teacher four years in a row would be enough to close the black-white test score gap" [3]
"Differences in this magnitude -- 50 percentile points in just three years -- are stunning. For an individual child, it means the difference between a 'remedial' label and placement in the accelerated or even gifted track. And the difference between entry into a selective college and a lifetime of low-paying, menial work". [4]
[1] http://www.amazon.com/Teaching-As-Leadership-Effective-Achie...
[2] Marzano, R. J. What Works in Schools: Translating Research into Action. Alexandria, Va.: ASCD, 2003, p. 7
[3] Kane, T., Gordon, R. and Staiger, D. Identifying Effective Teachers Useing Performance on the Job Washington, D.C.: Brookings Institution, 2004, p. 8
[4] Peske, H. and Haycock, K. Teaching Inequality: How Poor and Minority Students Are Short-Changed on Teacher Quality: A Report and Recommendations by the Education Trust. Washington, D.C.: Education Trust, 2006, p. 11