It is a location and that requires "estar"/"estamos". Estar: Position, Location, Action, Condition, Emotion(PLACE). Ser: Description, Occupation, Characteristic, Time, Origin, Relationship(DOCTOR). Oh I get what you mean such as, you graduated 20 years ago you are still in the same class(class of 1993). Not sure on that. Google translator gives "We are in the class of 1993" = "Estamos en la clase de 1993 ".
For anyone wanting to learn about DOCTOR & PLACE:
I believe that they have rectified the "her and I" issue, as you can see from the top of this thread where it says "she and I." Also, they are only interchangeable semantically, but, as far as translations go, Duolingo wants literal translations only. Therefore, no paraphrasing or semantic equivalents is ever accepted by Duolingo, for the most part.
«su» means "her" only as a possessive pronoun, as in "Her book was here." In "I gave her the book," "her" (indirect object pronoun) would be «le». In "I called her yesterday," "her" (direct object pronoun) would be «la». When there is a preposition involved right before the "her," it becomes «ella» again: "I gave the book to her." = «Le di el libro a ella.».