How do you make class possessive if there is only one class?

There are two accepted forms for possessive singular nouns that end with an s:
Add an apostrophe (') after the existing s at the end of the Word:

• one class'
  

Add an apostrophe s ('s) after the existing s at the end of the Word:

• one class's

Examples:

There is one class' trip scheduled for the tenth.

There is one class's trip scheduled for the tenth.