Susans' style is the best vs. Susan's style is the best

Susans' style is the best vs. Susan's style is the best

I don't know if I was hearing things but I remember my teacher having said apostrophes in words indicating possession are written with an apostrophe behind the "s", e.g. "Susans' style is the best", whereas according to this grammar article you have to write the apostrophe before it, e.g. "Susan's style is the best" and only write an apostrophe behind the "s" in the case of a plural-noun that ends with an "s" already.

Are the rules on these apostrophes universally agreed on?

The rules on apostrophes are very clear. In your example, the apostrophe must come after "Susan" and before the possessive "s". If your teacher really told you to use "Susans'", when referring to just one person called Susan, they were completely wrong. It can only be used if you're talking about two or more people called Susan. In that case, it would be "The/Both Susans' styles are the best".