님 in an honorific used when addressing someone such as a teacher, professor, doctor, or other professional. I do not believe the phrase 저는 선생님입니다 is correct. You don't introduce yourself in the honorific form. The correct way should be 저는 선생입니다. My students would address me as 선생님.
I've seen this in a couple of lessons, now. Can you please clarify.