Not to be an unsympathetic prick, but for all the times I tell you folks not to take low paying lowball jobs then you go and do it, ignoring me, claiming I'm crazy or something. It's your own fault for taking it. Stop taking low ball offers. As for the rest, start screaming back and correct him on what it means to be professional and tell him how dare he talk to you like that.
My guess is KT have gotten raises over the past decade or so while you didn't. Again, this is the case for some low hanging fruit best avoided. If more people would quit taking the shitty paying jobs, then they'd quit being offered. 9 to 7 is fine if your pay is in the 3.0's million per month. I think I recently got offered a 9 to 6 kindie type school for around 3 million plus. Maybe 3.1 or 3.2. I just commented on a job site Facebook group and got a private message. I already have a job so kept what I have.
If someone treats you that badly, tell them to screwoff. Quit if you need to. Get another job that pays more or get a 120 hour in class TESOL and go to Vietnam. Save a lot more money (cheaper cost of living). Either way, being overseas is hard enough. No need to take BS and accept low wages with garbage work conditions just because you watched a K drama on Netflix or saw BTS on YouTube. There's a whole world out there.
Anyways, hope you resolve the situation. You should all quit on them if they don't smarten up. Maybe Covid is forcing you to stay for a while. But next year, be gone.