In technical fields this is usually reliable (someone who went to CMU or MIT and graduated probably put in a pretty high baseline level of work), but in more liberal arts fields school quality correlates much less to a baseline quality level. At my alma mater (Amherst), for example, the easiest course of study would be to take 4 humanities classes / semester, spend 12 hours/week in classes, write 3 papers/semester for each, and then you're done. 50% of the student body does this. The hardest would be to double up on lab sciences (physics + chem, for example, or physics + neuroscience), which entails about 20 hours/week of classes, 30-40 hours/week spent on problem sets, 2 midterms and a final.
I've heard that Harvard is similar: the grade distribution at Harvard is roughly 10% As, 80% Bs, and 10% Cs. Getting an A at Harvard implies that you really did a lot of work, but getting a B could imply everything from "You really did a lot of work but just missed the cutoff" to "You learned basically nothing from your undergraduate education."