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.
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.