Dear Instructor:
Cases like this are exactly why the Asmodean system runs on threats, not rewards. Giving her "bonus points", rather than threatening to take away the points she's otherwise earned, even if the reason you're doing it is to get off on watching her beat others, will inevitably lead her to think she can slack off on her wizarding studies because you'll give her bonus points for fucking you. Which, in turn, leads to a fundamentally unhealthy relationship in which she's gaining bonus points and you're gaining sex, instead of you winning and her losing, at which point you might as well move to Osirion and worship Abadar.
Consider adding additional punishment sessions for idiots you're pretty sure are going to fail anyways, and letting your pet whip them to your own heart's content. Taking away the reward-of-punishing-others* that the best students have justly earned won't demotivate them if you otherwise put enough fear into them. But it'll make the most clever and Asmodean students in the class hate you, and even as a teacher that's not a smart place to be. As a headmaster, I'd be even more annoyed with a teacher who started faking his pet into the best performance bracket. This is something you could easily be caught out about, if one of the smartest and most Asmodean students had a favor owed with the headmaster - you won't be able to pass a test to see if your pet's performance was real.
(*) Two syllables of Infernal loanword.