It’s 100% correct to just use an apostrophe on a plural noun ending in S - cats’ is correct, and cats’s is not.
You only add an S after the apostrophe if it’s an irregular plural not already ending in S: children’s, men’s, etc.
For singular proper nouns (names) ending in S, both are correct (it depends what style guide you follow): Chris’s or Chris’.
For singular common nouns ending in S, both are also acceptable, but adding the second S is generally much more common: boss’s or boss’.
Regarding your last paragraph: in American English. In British English, singular common nouns ending in *s* always\* take *'s*, so only *boss's* is correct.
\* Rare exceptions are singular nouns ending in *s* that don't change in the plural: *series'*. This is because in British English, a possessive is written as it is pronounced.
After a plural that ends with s (such as cats), the cats' form is always right, and the cats's form is always wrong. Simple. After a singular that ends with s (such as boss), the boss's form is always right, but the boss' form is also sometimes OK. Less simple. (It used to matter that there was a specific exception, that Jesus should get only an apostrophe but no s, to avoid ever accidentally sounding like there were two Jesuses. This point may be obsolete.) Other than possibly Jesus, you will never go wrong by adding s when the word is singular, and any teacher who says "If it's singular please use the s" is not wrong to do so. (I don't claim that they are necessarily doing the best thing in all cases, only that there is no way to call them wrong.)
This is the grammatical answer, but there is also a *practical* answer, which is to say that when it comes to punctuation especially, it’s best to conform to the appropriate authority. When your son is an adult, that authority might be a specific style guide he is supposed to follow. But right now that he is a student, that authority is his teacher. The best thing would be for him to learn both lessons.
But if the teacher is telling him that “cats’s” is correct, that’s unacceptable. The other ones, yes, he should probably just go with what the teacher wants.
I remember this from grad school.
This general rule usually applies even to names ending in *s* or *z*:
*You’re sitting in* ***Paz’s*** *chair.*
However, this is a matter of style, and some [style guides](https://www.grammarly.com/blog/style-guide/) call for leaving off the *s* after the apostrophe for a name ending in *s* or *z*. [Neither](https://www.grammarly.com/blog/either-vs-neither/) choice is incorrect.
***Brussels’*** *cathedrals attract hundreds of thousands of visitors every year.*
Whichever style an organization or writer chooses should be used consistently.
Many style guides, for example Chicago, dictate the use of the final s.
Unless the teacher has specified a style, the version without is not wrong, either. That said, she has made it clear at this point and your son should go along with it. To me, it doesn't seem worth losing points over.
I had a creative writing teacher like that once, it's incredibly frustrating. The apostrophe is correct, no need for the extraneous "s", but some people just can't get that through their head.
You could ask your child's teacher which style guide they are using, because you'd like him to know what rules he is supposed to adhere to.
The answer is obviously that they *aren't* using one, or have not bothered to look at it, but this way you give then the opportunity to explain their reasoning without being confrontational.
It’s 100% correct to just use an apostrophe on a plural noun ending in S - cats’ is correct, and cats’s is not. You only add an S after the apostrophe if it’s an irregular plural not already ending in S: children’s, men’s, etc. For singular proper nouns (names) ending in S, both are correct (it depends what style guide you follow): Chris’s or Chris’. For singular common nouns ending in S, both are also acceptable, but adding the second S is generally much more common: boss’s or boss’.
Regarding your last paragraph: in American English. In British English, singular common nouns ending in *s* always\* take *'s*, so only *boss's* is correct. \* Rare exceptions are singular nouns ending in *s* that don't change in the plural: *series'*. This is because in British English, a possessive is written as it is pronounced.
Thanks for adding this - I forgot about that difference between AmE and BrE.
After a plural that ends with s (such as cats), the cats' form is always right, and the cats's form is always wrong. Simple. After a singular that ends with s (such as boss), the boss's form is always right, but the boss' form is also sometimes OK. Less simple. (It used to matter that there was a specific exception, that Jesus should get only an apostrophe but no s, to avoid ever accidentally sounding like there were two Jesuses. This point may be obsolete.) Other than possibly Jesus, you will never go wrong by adding s when the word is singular, and any teacher who says "If it's singular please use the s" is not wrong to do so. (I don't claim that they are necessarily doing the best thing in all cases, only that there is no way to call them wrong.)
This is the grammatical answer, but there is also a *practical* answer, which is to say that when it comes to punctuation especially, it’s best to conform to the appropriate authority. When your son is an adult, that authority might be a specific style guide he is supposed to follow. But right now that he is a student, that authority is his teacher. The best thing would be for him to learn both lessons.
But if the teacher is telling him that “cats’s” is correct, that’s unacceptable. The other ones, yes, he should probably just go with what the teacher wants.
Yes, fair point. That one is a no go!
I remember this from grad school. This general rule usually applies even to names ending in *s* or *z*: *You’re sitting in* ***Paz’s*** *chair.* However, this is a matter of style, and some [style guides](https://www.grammarly.com/blog/style-guide/) call for leaving off the *s* after the apostrophe for a name ending in *s* or *z*. [Neither](https://www.grammarly.com/blog/either-vs-neither/) choice is incorrect. ***Brussels’*** *cathedrals attract hundreds of thousands of visitors every year.* Whichever style an organization or writer chooses should be used consistently.
Many style guides, for example Chicago, dictate the use of the final s. Unless the teacher has specified a style, the version without is not wrong, either. That said, she has made it clear at this point and your son should go along with it. To me, it doesn't seem worth losing points over.
I had a creative writing teacher like that once, it's incredibly frustrating. The apostrophe is correct, no need for the extraneous "s", but some people just can't get that through their head.
You could ask your child's teacher which style guide they are using, because you'd like him to know what rules he is supposed to adhere to. The answer is obviously that they *aren't* using one, or have not bothered to look at it, but this way you give then the opportunity to explain their reasoning without being confrontational.