Here's why:
* Common nouns refer to general things or people, not specific ones. They are not capitalized.
* Proper nouns refer to specific people, places, or things and are capitalized.
"Class" can refer to a group of students, a social group, or a category. It doesn't refer to a specific, unique entity.
Examples:
* Common noun: I'm taking a history class.
* Proper noun: I'm taking a class with Professor Smith.
Let me know if you'd like to explore other examples!