Technically, it would be "It is I."
But, no one says that. Today, people say "It's me".
In fact, if someone knocked at my door and replied "it is I" after I asked who was there, I probably wouldn't open the door.
(1) As the teachers have told you, most people nowadays would use "me."
(2) This non-teacher respectfully suggests that you follow the rule:
It is I, he, she, we, they.
I have to disagree with you there.
As you say, most people use 'me' - it is now the accepted, natural usage for all but a minority of speakers. Most people learning English want to sound as natural as they can, not to sound like a walking grammar book.