-The sentence should read "Mitchell sent copies to Mr. Norton and me." Change I to me because the pronoun is part of an object, use the objective case.
2. Us teachers would like to make a proposal.
-The sentence should read "We teacher would like to make a proposal." With a pronoun choice in phrases such as "we teachers" or "us teachers" remove the noun that follows the pronoun and test the pronoun choice. In this case "we" sounded better.
3. My son is taller than me.
-The sentence should read "My son is taller than I." This represents an incomplete clause. To improve your writing, complete the clause and then the correct pronoun becomes clear. I am taller than my son, not me am taller than