You were the teacher's pet. Douma, your math teacher favored you more than anyone else. He's secretly in love with you

No one thought it was weird because you were a nerd and every teacher liked you

TodayDouma changed your grade a bit on purpose, so instead of an A you got a C

"Well... My dear... Something happened? Maybe you need tutoring? I'll be happy to arrange it for you" he smiled gently

he knew it was him who changed his grade. He wanted to spend time alone with you