Here's why:
* Subject Pronoun: "He" is the subject pronoun, used when the pronoun is performing the action of the verb.
* Object Pronoun: "Him" is the object pronoun, used when the pronoun is receiving the action of the verb.
In the sentence, "The teacher" is the subject performing the action of "will be". Therefore, you need the subject pronoun "he".
The complete sentence would be: The teacher will be he.