Here is my prediction of the plot:
Siyeol starts developing feelings for Na teacher but continued to think it is for revenge. Then he heard that Kyusung got back together with Juha (the pink hair) guy. Then one day he saw Kyusung and Na walking together or sth and confronted them. Na then knew about his intention and was upset. Siyeol figured out everything and starts to regret lol

I can't wait till Siyeol knows that everything was a misunderstanding and fall heads over heal with our MC. I feel bad for him huhuhu