The possessive form of the singular noun senator is senator's.

Example: I got the summer job as a senator's intern.

The possessive form of the singular noun grandma is grandma's.

Example: I brought flowers for grandma's birthday.

The plural form for the noun potato is potatoes; the plural possessinve form is potatoes'.

Example: The potatoes' jackets were all pitted.

There are two accepted forms for possessive singular nouns ending in s:

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

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

Examples:

Russ' bicycle is new.

Russ's bicycle is new.

There are two accepted forms for possessive singular nouns ending in s:
Add an apostrophe (') after the existing s at the end of the word: Weis'
Add an apostrophe s ('s) after the existing s at the end of the word: Weis's

Examples:

Mr. Weis' classroom is number twenty six.

Mr. Weis's classroom is number twenty six.

The plural form for the proper noun Susan is Susans; the plural possessive form is Susans'.

seniors'

add number before adding possessive.

senior >> seniors (plural)

seniors >> seniors' (plural possessive)

+s' works for most things except for irregular plurals:

child >> children >> children's

woman >> women >> women's

Also, words with -ies plurals, just add '

country >> countries >> countries'

The plural form of the noun class is classes.

The plural possessive form is classes'.

Example:

All of the classes' language requirements can be found on-line.

The possessive form of the noun grandfather is grandfather's.

Example: I'm spending the weekend at my grandfather's farm.

The plural form of the noun year is years.

The plural possessive form is years'.

example: I have five years' experience as a chicken plucker.

No, it is singular, the possessive form of it is its. The plural form of it is they or them, and the possessive form is their.

To answer the question directly: there is no such word as ITS'.