he's 20 but in high school (am in chapter 11 btw)?????

college not highschool

I could totally be wrong so don't quote me, but I do believe they're using the Korean age system. When babies are born their time in the womb is automatically counted as a year passed, so they're 1 at birth and then again once new years passes, everyone gets another year added, so people in Korea are +2 years or so compared to international ages. (The system was gotten rid of as of this year I think)