Examples:
The students argued against more homework.
This means more than one student argued against more homework.
The student's arguments for a higher grade proved effective.
This means one student argued successfully for a higher grade.
The students' backpacks were equipped with books, paper, and writing utensils.
This means more than one student had a backpack, and that each backpack had books, paper, and writing utensils in it.
Possessives when the name ends in the letter "s."
A general rule is to spell it as you would pronounce it.
Examples:
Sarah's books.
One person named Sarah had some books.
Socrates' books.
One person named Socrates had some books.
The Lucas's car.
This likely refers to more than one person with the last name Lucas who jointly own a car.