Is a teacher a pronoun

No, the Word 'teacher' is a noun, a Word for a person.

A pronoun is a Word that takes the place of a noun in a sentence.

Examples:

The teacher gave me an A. She said that I had really improved. (the pronoun 'she' takes the place of the noun 'teacher' as the subject of the second sentence)

The teacher said that I could give my assignment to him on Friday. (the pronoun 'him' takes the place of the noun 'teacher' as the object of the preposition 'to')