Story

When I was in college, the school would kick you out of the program if your GPA dropped below a certain threshold in the core classes (science, math, etc) and higher threshold for classes in your field (computer science for me.) Furthermore, the thresholds where quite high, 3.0 and 3.5, I believe.

The school allowed anyone to "drop" a class without any GPA penalty if done by a certain date. This led to the mass-abandonment of the harder classes midway through the semester. After this date, a lecture hall of 300 calculus (or whatever) students instantly became less than a hundred.