However, if I tried to leave the school at 16:59, he had a hissy fit, threw a temper tantrum, and had a complete meltdown. Always the first in the staffroom, I was also often the last to leave it.

Isn't that always the way?

Bullies, and yes including chinese and koreans amongst them as some of the worst, if not the worst, often pick on those they fear. Perhaps they thought you a tall poppy to be trimmed down, very common in confucian societies where individualism is seen as some kind of evil.

You are far better out of that place for sure!