Here's why:
* Common nouns refer to general categories of people, places, or things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things. They are always capitalized.
Examples:
* Common noun: dog, city, teacher
* Proper noun: Fido, New York City, Ms. Smith
Since "students" refers to a general group of people, it is a common noun.