Go to the link above - the NY Times has some AMAZING graphs on how grades have gone up, up, up over the last 40 years. It is staggering.
ONE possible explanation, in my opinion:
1. Private schools offer more 1-on-1 instruction and greater access to the Professors, therefore allowing students more tutoring access. They also have smaller class sizes, therefore allowing more evidence-based teaching methods to flourish and allow students to understand the material better.
That's it, that's all I've got and that only explains why private schools may have higher grades than public. Why grades have shot up overall is a study in the pressures Administrators place on Faculty to keep the numbers up...and that's a whole post for another day! For now, check out the startling evidence in the graphs.