There are also outliers, about which it interesting to ask "why?" This [1] example of poor kids in a good school was recently reported in Australian media. The Albert Street School [2] in Johannesburg is run by Zimbabwean refugees for Zimbabwean refugees. Recently, it's been achieving results comparable to the best African private schools.

[1] http://www.abc.net.au/news/2011-11-06/disadvantaged-students...

[2] http://www.eliasfund.org/partners/albert-street-school