Su can mean his/her/your (formal)/their/its. Su agrees in number with clase (su clase or sus clases). Her seems to be most likely choice for this sentence because she must be in the class to be the best of the class. Your would make sense if you were talking to another member of the class the teacher of the class.
"Of her class" sounds like you're talking about a boat or something. "In her class" sounds better (to me) if you're talking about a class made up of students.
su can mean your as well as his/her/its. so this should be fixed to include "she is the best of your class"
Your should be right. It didn't specify whether it was her class or usted's
Well Maybe I am wrong, but I am pretty sure that in Spanish you say "la clase" to translate "the classroom" as well as to translate "the class", like we do in italian, with "la classe"
You misunderstand. I wasn't querying the vocabulary, I was commenting on why your choice on this occasion would be wrong or at least so unlikely an option that it would be a poor translation. "Best of the classroom"? I think it is reasonable to word it in English as "best IN the class" as that is more commonly said, I believe, but as the classROOM is a place I think you would have to use "en" to mean IN. And in any case it is an odd observation.
I wrote "She is the best....." but I think I have also heard "mejor " used to mean "older" as in "mi hermano mejor" for "my older brother".
You probably know this by now, but I think you meant "mayor" for "older" (mi hermano mayor)?
Doesn't the sentence sound better as, "she is the best out of the whole class."?