The possessive of plural nouns ending in s is formed by adding an apostrophe after the s. For example, if you are referring to books belonging to your two teachers, you would write, “my teachers’ books.”
But in terms such as teachers union and farmers market, in which the first noun is plural and ends in s, some writers omit the apostrophe after the s because the first noun is attributive—that is, the first noun acts as an adjective rather than as a possessive noun. Although this practice is not incorrect, the MLA follows The Chicago Manual of Style and only omits the apostrophe for proper nouns such as Teachers College or Department of Veterans Affairs (“Possessive”). Thus, we would write teachers’ union and farmers’ market.
For those who prefer to omit the apostrophe in generic terms, be aware that if the plural form of the first noun does not end in s, an apostrophe must be used:
the sheep’s meadow
“Possessive versus Attributive Forms for Groups.” The Chicago Manual of Style Online, 17th ed., sec. 7.27, U of Chicago P, 2017, www.chicagomanualofstyle.org/book/ed17/part2/ch07/psec027.html.