Humiliate her.

"Okay, Emily," you begin," you can suck my dick only if you do everything that I ask of you." Emily looks a bit nervous, but still graciously agrees, willing to do anything right now.

You feign thinking for a moment, even though you already know what you have planned for her. "Alright. I've decided on what you must do."

"What...what is it John?" She asks.

"Well, you just need to prove to me that you deserve to suck me off." You answer.

"How am I supposed to do that."

"Okay, Emily, let me elaborate. First, your cock sucking must be top-notch; I only go for the best. You must suck at least 15 guys, and be sure to beg them for the privilege first. Next, I want you to prove that you are actually deserving of my dick. You will make sure not to engage in intercourse or masturbation until I say so. In fact, you will wear a chastity belt and give me the key. Also, you must always wear your sluttiest clothes. Finally, you are also going to eat out 10 girls for practice. You must beg them for it, also."

Emily looks at you and nods with understanding, showing her willingness to do anything for you. The time between classes is running short so you head towards your respective classes.