Safe - Sane - Consensual
Master David
Doing a Deal With the Professor
Your grades in one important class are suffering, and you are afraid of what will happen if your parents find out. You decide your best option is to beg your professor to cut you some slack - you're desperate enough to do anything he asks.
After hearing your story, he makes you an offer you can’t refuse. You will become his bitch, and service him whenever and however he wants. In turn, you will get at least a passing grade.
Variation 1: The professor can also provide “motivation” by telling you that anytime you hand in a paper with less than a C grade, you will be flogged and whipped, or any other punishment you want to me to work into the scene in addition to being my bitch.
Variation 2: Instead of professor, it could be any authority figure who holds something over you - a neighbor, boss, policeman, etc. 
(Obviously, the type of services you offer the professor or the punishments involved must be agreed between us beforehand.)