Hi toby again
As panic button said, it is actually correct with singular nouns ending in an s to do it either way, so you can add a 's or an apostrophe alone. So you could argue with the teacher marking it wrong, it is not wrong, it's just not the way they do it.
Weirdly after a bit of research on this it would also seem the exception is religious names when it has to be an apostrophe on it own, such as, Jesus' strength, Achilles' heel.
How the hell did punctuation get so damm complicated :)