I am in an argument with a sign maker at the high school I'm building. I say it should be Men's Locker Room. He said it's Mens' because Men is plural. Same with Women's Locker Room versus Womens'. Who is correct?
You are, and here's why:
In very old English, we formed plurals not with "s" but with "en": one brother, two brethren; one man, two men; one woman, two women; one child, two children; one chick, two chicken (no kidding!).
Some of those words have stayed in the language. They're plurals, but when we form a possessive of them, we treat them as singular:
Women's locker room
It would be especially embarrassing in a school to have the apostrophe in the wrong place, so stick to your guns on this one!