Do you ever mix up "who's" with "whose"?
The trick to remember is "who's" is a contraction that means "who is" or "who has" -- that's its only usage.
In your questionable sentence, try swapping out "who's" for "who is / has" and see if it still makes sense ("Who's the teacher?" -> "Who is the teacher?").
If it doesn't make sense, then you actually want to use "whose" (which is used to show possession).
The trick to remember is "who's" is a contraction that means "who is" or "who has" -- that's its only usage.
In your questionable sentence, try swapping out "who's" for "who is / has" and see if it still makes sense ("Who's the teacher?" -> "Who is the teacher?").
If it doesn't make sense, then you actually want to use "whose" (which is used to show possession).