Generally, we don't refer to people as it. (Of course, I do - of babies especially, but I only do it to irritate mothers everywhere)
There are valid exceptions.
"Who is it?" is a perfectly valid question when you hear a door knock or are asking your son who they are talking to on the phone. The response is, "It's Bill."
And when we point to pictures of people, you can use their gender-specific pronoun, or "that" because you're not really pointing to the person. You may also point across the street at someone and ask your companion, "Who is that?" The only impolite thing about that was pointing in the first place. And then the respnse can be "That's my teacher" or "He's my teacher" - either is fine.
And as for babies, when you don't know their gender, the more polite thing is to simply refer to it as a baby, "Oh, look at the baby!" and then ask, "Your baby, is it a boy or a girl?" (notice how I slipped an it in there? That one is fine.) And after you know the gender, use the proper pronoun.
Jlinger: You explained to Pedro about the use of 'it'. To then say, You've had "the lesson" - now why don't you tell us which you think are correct. I get scolded here for doing students' homework.
...is rather pointless, since logically, removing 'it' from the examples leaves one:
Who is he?
-He is Mike.
The point is, how colloquial is this, and are there other ways of expressing this?
Don't let one admonition inhibit you, but rather get your juices focussed.