basically both characters are fucked up lol. sensei has this pent up anger and resentment bc he always had to be a perfect child, and mirai has mommy/abandonment issues and trauma from being raped by his dad. sensei visits mirai as a concerned teacher but as he later revealed, it was actually entirely selfish since he immediately and always thereafter lusted for mirai. when mirai tried to commit suicide, sensei is angry bc it's not fair that one of them gets to escape their circumstances. in retaliation, mirari comes back to school and harasses/abuses sensei. this is seemingly pure revenge at first, but we later learn that it is actually because mirai has grown dependent on sensei. he has no one in his life. he wants someone who cares about him, (which sensei said he did just before he raped mirai), even if it's fake. it ends with them happily running away together to start afresh their questionable relationship. i may have missed something but that's what i took away from the manga.
probably bc of the main characters' past (rape/abuse). the manga itself isn't necessarily 'tragic' (i.e. it ends okay), but the characters' pasts are tragic and we get snippets of that, therefore the tag.
Can someone roughly explain the whole story? Im just in a state of confusion~