In the comment thread of this post I rather glibly stated that Teach for America had a dropout rate of ~50% (n.b. that I was defending TFA at the time). Turns out it's more like 15% for two years, though it gets a bit murky after that. Regardless, I still believe that TFA is a worthwhile program with a limited scope.
The dropout rate for Peace Corps in South Africa, on the other hand, is actually about 50%. This is higher than average--according to this Peace Corps wiki, the overall rate is about 33%. I find this discrepancy unsurprising.