Here's why:
* Proper nouns are specific names of people, places, or things (e.g., John, London, Eiffel Tower).
* Common nouns are general names for people, places, or things (e.g., boy, city, building).
"English teacher" is a common noun describing a general occupation. It refers to any person who teaches the English language.
If you were referring to a specific English teacher, you would use their name, which would be a proper noun (e.g., "Ms. Smith is an English teacher").