What is the possessive form for Stevensons?

If the proper noun Stevensons is a singular form, the singular possessive form is Stevensons's.

example: Mr. Stevensons's classroom is at the end of the hall.

If the proper noun Stevensons is the plural form, the plural possessive form is Stevensons'.

example: The Stevensons' house is the one with the blue shutters.