The possesive form depends on the noun you want to change into a possesive form.
If the noun does not end with s then you add ‘s (apstrophe s):
dog – dog’s boy – boy’s
If the noun ends with s (mostly in plural forms) you add only apostrophe:
boys – boys’ teachers – teachers’
As you see sometimes both forms are possible (boy’s – boys’) but they have different meaning.
The form ‘s indicates a possesive form of a singular noun (boy’s = of a boy (one person)).
While the form s’ indicates possesive form of a plural noun (boys’ = of boys (more than one person))