I work in an international school in Jeju.
I work at an international school in Jeju.
I work for an international school in Jeju.

In order for you to pick up your ID card when I am not in the room, I will leave your card on the small window of the door.

Seoul had a local referendum on/about whether it would provide free school lunches to all students.

The result of the referendum was opposed to the mayor's plan.

The allegation that he was involved in/with some real estate speculation/speculations was proved by many witnesses and proofs.

He was with some of his wife's family members. **