Excellent article in Salon by David Sirota that appeared in June. Not sure that I read it at the time; several friends of mine have pointed it out to me recently, so I thought I would return the favor to my readers.
Sirota points out that the strongest socio-economic correlation anywhere in the US is between family income and educational achievement.
I would add that in my town, Washington, DC, that correlation still occurs, yet ALL of the teachers in ALL of the schools are either brand-new (that is, hired by DEformers Rhee and Henderson under mayoral control) or have been rated effective or highly effective. So the low achievement of poor kids in Washington, DC can not be laid at the feet of the imaginary legions of lazy, unionized teachers waiting for retirement!