Who says it's a conspiracy? It's more a matter of culture. I have seen it happen: there's a trend in the class where the homework grades, on average, are high,

Who says it's a conspiracy? It's more a matter of culture. I have seen it happen: there's a trend in the class where the homework grades, on average, are high, but the exam grades are low, even though the material is mostly the same. This happens at reputable schools and at "low tier" schools.