Identify Help finding a OVA or Doujin (I don't really remember the source)

[Minuit]

Hello, I have a hentai (I don't remember exactly if it's a manga/OVA/doujin) in my head since some days and I can't find the damn thing, this is driving me insane.

From what I remember, it takes place in a school with 2 students (a girl & boy) compete for the best test score (or midterm score) and either:

• the one with the best score can ask anything to the other;
• the girl challenge the boy to get better points and if he do, he can do anything to her

then another guy (NTR) see this and manage to beat them both & blackmail her into doing stuff to him too.

It wasn't anything exceptional but I can't get it out of my head... I hope anyone can help me with this.