Just what I was taught in my English lit classes years ago. If I'm wrong, I apologize, and I lay the blame for my mistake at the feet of two people: My English teacher for getting it wrong, and me for not correcting it.
As I said, it has been a long time since I have read the play. I have a 1938 edition of Shakespeare with an introduction to
Othello by Thomas Marc Parrott, Professor of English at Princeton, and he says nothing about Iago being Jewish, and there's nothing in his introduction to
The Merchant of Venice to suggest that Shakespeare had another major character in one of his plays who was presented as Jewish.
Perhaps some clever English scholar has detected some coded references in Othello which revealed Iago to be Jewish, or perhaps your English teacher was misinformed.