This is my latest pet peeve. Don't schools teach this anymore? I am astounded by the number of times I've seen these used incorrectly in books and on professional blogs.
Here's a brief review:
Ownership by one person or thing: school's John's
Ownership by more than one person or thing: students'
Using its and it's:
When used as a possessive, use its: The truck lost its muffler
When used as a contraction--"it is": It's best not to question a judge in a courtroom.
When a word ends with the letter "s" (including a plural) to make it possessive use an apostrophe after the "s" without adding another "s": snakes'
How hard is that?