Here's why:
* "Yearbook" is a common noun: It's a general term for a book containing records or information about a year.
* Proper nouns are capitalized: Proper nouns refer to specific people, places, or things.
* "Class yearbook" is a specific instance: The term "class yearbook" refers to a specific yearbook belonging to a particular class.
Therefore, you would write it as "class yearbook" or "the class yearbook".
For example:
* "The senior class yearbook is filled with memories."
* "They're working on the class yearbook."