Possessives are so tricky until you really learn the rules, and even then it is easy to get confused by so many irregular nouns. Possessive apostrophes are different from regular apostrophes, and they are only used to show possession.
The general rules are as follows:
To form the possessive of a singular noun, whether it ends in an -s or not, add an apostrophe and an -s. For example, "this is the woman's room," "that is the bus's route," or "this is the man's closet." The rule for singular nouns is not difficult to remember--always add an apostrophe and an -s.
Plural nouns are not difficult either, though there are two rules to remember. If a plural noun already ends in an -s, just add an apostrophe. For example, "the boys' room" (which means the room belongs to more than one boy), "the ladies' room" (which means the room belongs to more than one lady), or "the flags' stripes" (which refers to the stripes on more than one flag). To review from the Purdue Online Writing Lab:
- add ' to the end of plural nouns that end in -s:
two cats' toys
three friends' letters
the countries' laws
If the plural noun does not end in -s, add an apostrophe and an -s. For example, women is plural and does not end in an -s, so this is the plural usage: "the women's room." Men is another plural noun that does not end with an -s, so the possessive is "men's," meaning something belongs to all the men. To review from the Purdue Online Writing Lab:
- add 's to the plural forms that do not end in -s:
the children's game
the geese's honking
Now for your question.
- One student's locker is open (a locker belonging to one student)
- The students' lockers are open (every locker belonging to the students is open)
So, the possessive form of students (a plural noun that already ends with an -s) is students'. Think about the apostrophe as being a clue to the reader about number; in written work, the only way a reader knows whether or not you mean the possessive of one student or more than one student is by your use of the apostrophe.
The resource which I linked below is a very good tool for this and other grammar/punctuation questions. There are a few more rules about possessives, but you do not need to look at those unless you are interested.