Translation:Who is in the same class?
I believe it's incorrect because クラス doesn't refer to a course, it refers to a graduating class/grade level. so literally this means "who is the same (graduating) class?" but it's translated here as "in the same class" to sound more natural in English
It does not have an "in". Literally speaking, the sentence means "who is the same class?". As the class is made of people, people are (part of) the class. It's just that English and Japanese have different ways of saying some things, so literal translations will often fail
ですis a multipurpose verb and it can mean anything depending on the contex. 同じクラスですmeans to have the same classes, classmates, to be at the same level. Were います to be used here, it would mean someone physically existing in the same class at that very specific moment.
Any reason why "Who is in your same class" is incorrect? Duolingo is vague on context so often I can never tell.
"your same class" doesn't really make sense to me. I guess you could say "who is in your class" but then you would have to leave out 同じ(おなじ), which is wrong.
Lacking any further context, as questions default to second-person in Japanese, the meaning is "Who is in the same year of schooling/(to-be )graduating class as you( are in)?" (クラス means 学級 and is closer to the original sense of 'class'. Incidentally there's no tense and they seem to have the sense of still being in the same class even after they've graduated.) This seems redundant in English and generally gets reduced to "Who is in your year( of schooling/school)/grade/form?" (As if you own it...) Now how would you normally state that in Scotland, say? It's ironic I don't do Scotland, as my father was born there . . .
In the Japanese sentence, nowhere. It literally translates to "Who is the same class?" It's just that English and Japanese phrase that question differently (English uses an "in", Japanese doesn't), so a literal translation from Japanese to English would sound odd if not outright ungrammatical.