What is Grade Mule?
1.
As in "riding the grade mule."
One that rides the grade mule cares not about how much he or she absorbs and/or takes back from whatever class; they only care about getting a grade.
Think of it as a mule led by a carrot and stick.
Benny: "My assignments six months late, but I'm still getting 25% credit. It's better than a zero."
Phil: "Fool, you better get off that grade mule."
See