It's been a round a long LONG time.
Bellcurve - an assumption that based on any particular teacher or subect, a certain percentage of the class' student are expected to get with a range of marks (lets say 60% of the class will get between a c- and a b+; 30% will get either between d+ and d- or between a- and a; and the rest will either get superb or fail horribly).
If the actual results don't fit these results...the entire classes' marks are adjusted higher or lower until it does.
This is based on the assumption that if more people got on the higher end than was expected, then the teacher was too lenient and gave marks away too easily, and if too many people got lower, then the teacher was too strict.
It's usually followed up with a comparison of test questions/assignments and samples thereof from the class in question before any marks are adjusted.
Voila! A bell curve!