Here is a chart I just threw together to help explain a point that most of my students--particularly underclasspersons--find difficult to comprehend. (Note that the percentages I've chosen are estimates based on my own experience. I use them for ease of interpretation, not because they are an accurate representation of any population.)
Most college students--at least the ones here at a small, wouldbe-elite liberal arts institution--were accustomed to getting A's and B's in high school. Once they get to college, though, simply based on the elevated demands and expectations of higher ed., the grade distribution is reshuffled, and suddenly, some folks who were used to A's and B's find themselves earning C's, D's, and F's. College is not high school.
UPDATE (7/14/11): New research covered by Economix and Inside Higher Ed.