Think of it like this. A teacher is a person who teaches. A job is something a person does. "My job is a teacher" is incorrect because a job isn't a person who teaches: YOU are the teacher. It might work abstractly if you're trying to imply your job teaches you many things, but even then it sounds awkward. You can say "My job is to teach" though.

You can say "I am a..." to introduce your position, and "My job is to..." to introduce your responsibilities. "My job is to treat patients. I am a doctor." "His job is to teach teenagers. He is a High School teacher." "Their job is to sell houses. They are Estate Agents."

No. I always say "I'm a college student" If I had a job, I'd say "I work as an officer" or "I'm an officer" I don't know "my job is...." is right or not but it's not sound natural for me

what else job do you know? it is incorrect, isn't it?

I work as a doctor and my job (mission) is to make correct diagnosis and to give the patient good therapy.