What is proper English singular possessive form for the word 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:

• class'

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

• class's

Examples:

My class' trip to the museum is scheduled for Friday.

My class's trip to the museum is scheduled for Friday.