Students in Literacy Boost classroom attended school more often than students in the control group. Even more encouraging, the gains were biggest among students who were furthest behind. Literacy Boost can even help compensate for huge class size: in Malawi, the program works just as well in the 175-student classroom as the “small” 75-student classroom. In schools outside the program, by contrast, achievement drops as class size rises.
That is from this very good article by Tina Rosenberg in the NYT.