
I have a theory, could be a spoiler too.
So after reading the comments and such I think that the MC is imaging all.
His dad isn't bad, the MC is punish himself (with the can), the friend he made isn't real and (maybe) his grades aren't decreasing.
It's all because his mom left and he still think he is at fault.
His rival could be real or not. But I think he isn't real too. He is just there for a reason, why his grade are dropping and that he isn't first place anymore.
Maybe I'm wrong, feel free to share you thoughts (sorry for my weird English)
Ninotaku