I guess elementary schools don’t teach the same things nowadays that they did when I was a youngster. Of course, that was many, many decades ago, but I thought there were some subjects that were eternal. Like how to form the plural of nouns. Judging from this excerpt from Yahoo! Style, schools are neglecting that bit of knowledge — or the writer was playing hooky that day:
The possessive plural of lady is ladies’. It follows a simple rule: Form the plural of the noun and if it ends in S, add an apostrophe. So, it’s idiots’ and dummies’. If the plural doesn’t end in S, add an apostrophe and S: So women’s, children’s, and alumni’s are correct.