The factor that concerns me is that the circumstances causing you to sell the home have to be not just unforeseen, but unforeseeable. That's a higher hurdle.
Section 1.121-3(b)(5), included in what Mike9241 quoted above, says "The circumstances giving rise to the sale or exchange are not reasonably foreseeable when the taxpayer begins using the property as the taxpayer's principal residence." [emphasis added]
Section 1.121-3(e)(1) says "A sale or exchange is by reason of unforeseen circumstances if the primary reason for the sale or exchange is the occurrence of an event that the taxpayer could not reasonably have anticipated before purchasing and occupying the residence." [emphasis added]
It's not clear to me that you meet this requirement. The wording of the regulations suggests to me that it requires something that happened to you because of external forces that were not under your control, not something that you chose to do. When you bought the home, did you really have no idea that you might go to graduate school? If you even thought about it, it was not unforeseeable.
But your situation is unusual and unclear. I agree with Mike9241 that you should consult a tax professional.
