Your Mommy or your teacher have made a deal with you: if you get good grades this semester, you’ll get a blowjob. With motivation like that, how could you not get good grades?
I had originally envisioned this as Mommy hiring a local college girl to give it to you, but it could be Mommy herself who gives it to you, or your big sister, or your teacher, or a celebrity, or really anyone.
There could be a non-con element to it: your big sister is ordered to give you a blowjob, even though she doesn’t want to.