Embassies aren't required to accept cosponsors if the interviewing officer determines that there is a greater risk of you becoming a public charge based on your fiancee's employment history, or lack thereof. Unless there are other valid reasons for her not having a job, the embassy may deny the application based on that fact. Studying may be a valid reason but it is completely up to the IO.
One option is figuring out if the embassy will accept the i864 instead of the i134. Another is for your fiancee to get a job and show some sort of consistency in her income to support herself(and you). The third option is getting married and applying all over again because the i864 is a binding contract. However, I'd go with the second option because your fiancee will have to submit a new affidavit of support by the time of AOS. Joint sponsor are accepted there, however unless she or you get a job very soon(in the US), you can't support each other financially.
