James' or James's?

[MamOfTwo]

My Yr 2 child has been learning about possessive apostrophes in class today. Her name ends with an 's'. Teacher has told her it should be James' but I think it should be James's. (Her name is not James, obviously.) I know both are correct, but is one 'more' correct? And if so, why?

I was taught james' when i was at school say it out loud james-s book rather than james book

Darn it. I didn't see this thread and started a whole new one on exactly the same topic. Sorry