there's a variety of things he has overheard in regards to why he's attending panbakmoon: first, it was a prank gone wrong, then it was bullying, and then it was something outrageous like getting caught with the headmaster's own kid. he thinks it's funny, something he can laugh about when met with the truth of the matter. which is a lot more simpler than something like explusion.
hyunjin was never a rich kid, but he was attending a rich school. sure, the scholarship he was given for being an active member of the school's swim team did wonders for the tuition costs, and yeah it was nice to be able to say he was going to a private school his family wouldn't have been able to afford otherwise. things were fine, until his mother got sick. the cancer had came back. so his father works longer days, his mother is often in the hospital and hyunjin's left to his own devices as the family tries to stay afloat.