Behavioral motivation can be overdetermined -- in real life it often is. But for the purposes of the logline, where brevity is a requirement, you have to decide what goal is the most important, the one on which all the others depend.
Would your character be happy and content if he only fulfilled or exceed his writing assignment -- got the coveted "A"? I'm guessing the answer is no. If that is the case, then it's merely the trigger motivation for the inciting incident.
So that leaves the act of sabotage, peer acceptance and getting the girl. Having peer acceptance is a really big deal at that age. So is getting the girl. Maybe he thinks he can have it all. But what if he can't?
What if he discovers during the course of the story that he can win peer acceptance or he can win the girl -- but he can't win both? What if he can only win the girl by not going along with the sabotage because she's a decent, principled person? Indeed, what if he must sabotage their sabotage forfeiting all hope of peer acceptance if he is to have any hope of winning her love?
In other words, trap him on the horns of a dilemma.
fwiw.