One of the first thing applied researchers learn is that causality in applied research is very difficult to establish. Things are more about “here is my story” and here are the data I can offer in support of this story. Full disclosure is important because other stories are often possible. You do not lie with statistics, but with your interpretation of the method that generated the statistics.
It appears that KIPP may be successful simply because those who cannot meet the expectations leave. I thought it was supposed to be successful mostly because students worked longer (which also is not that surprising, but a useful conclusion). So, it looks like working longer leads to a high drop out of lower achievers. So, now, you really have a situation in which making causal statements is difficult. You cannot even claim that longer time commitments produce better letter. Maybe, it just is a way to get rid of your low achievers.