Which is the correct way to rewrite this phrase? my class's garden my class' garden my classes' garden my classes garden

class's = owned by one class.

classes's = owned by multiple classes. (usually shortened to classes')

You can't shorter class's so class', however. Here's why:

You can only shorten it when it's a plural noun that ends in an s or when it's a proper noun (names, places) that ends in an s.

So unless there's more than one class, it's class's.