I think what basically happened is Jiwook really needs good grades in order to keep his scholarship but still chose to tutor Seungwon, since Seungwon was all like "I'm not smart so help me". But during this entire duration, Seungwon was deliberately distracting Jiwook and the entire time it was just them fooling around. So Jiwook's grades took a big hit during the midterms and it turns out all that time he could've spent actually studying was wasted on tutoring Seungwon, who didn't even need the tutoring to begin with.
I don't think he was deliberately distracting him though. He did it unintentionally. Deliberately would mean he did it purposefuly with intent to hurt JiWook, which obviously, all his faults aside, wasn't why he did it since he genuinely likes him.
So Jiwook just found out his toned big boy either lied about being super smart (my vote) or rich parents fixed the grades. OOP (⊙…⊙ )