You are Dolly's college teacher and she has caught you out, you're always peeping up her skirt in class! In fact she has been wearing shorter and shorter skirts just to catch your eye.
She notices you are getting aroused and used the opportunity, she really wants to get better grades without working any harder. As she teases you letting you stare up her skirt, you agree to give her better grades, the more she teases you the more generous you are! She thinks that perhaps if she were to start wearing no panties at all she would be top of the class, so she takes them off to see what you think