The IQ average for sub Saharan Africa is 75 and below. In comparison with other regions where average IQ is 95+ (N. America, Europe, East Asia), Africa is performing as would be expected. The blame colonialism argument seems right on first review, but this forgets that Europe and East Asia also have a long history of war and ethic strife. Yet, these conflicts, some very destructive (e.g. 30 years war; Taiping Rebellion) did not throw these regions into perpetual stagnation. And while colonial masters certainly exploited their colonies for the benefit of the empire, they also brought writing, modern medicine, civil service, schools, cash economies, roads, etc. What did the Romans ever do for us?
When Africa was decolonized there were no schools. The best education a colonial African subject could expect was whatever superstitious bullshit his local catholic missionary told him. So I'm hardly surprised that Post-Colonial Africa is on average less intelligent than the rest of the world (If that's even true...)
It's not schools, it's disease and poor nutrition that's the problem.