To say someone (usually female) has bad grades is a euphemism for saying they have
large breasts.
For instance, a girl with
D's (really bad grades) has really big breasts. You can say the worse the grades someone has, the bigger their breasts.
Did you see the new girls in
English class, she had really bad
grades!
I've never seen someone with such bad grades!
Have you seen Jessica lately? Her grades have been getting
worse and worse.