Something you will always get if you are looking them up right now. These are grades below a certain acceptable level

When someone goes to the gym and spends 90% of the time on their phones scrolling through social media

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.