My guess is that he suddenly disappeared because their parents died. Either he needed to work to take care of his younger bro, or they went to live with a relative.
I'm guessing sensei is gay. It explains the lack of attraction and it would fit with him understanding what it's like to be bullied. And it's always nice to see a gay character introduced that isn't a blatant stereotype.
Kinda hate the male MC. Pretentious dick. "Can't learn anything from your phone." My ass. A phone is a MORE efficient way of learning. You can read your damn books on your phone. Fucking boomer logic.