Most likely the answer is Nicholas Cage.
Then again, it sort of depends on how *soon* after the death actually occurred as to the people who went upstairs (or were already there at the time).
According to published reports, it was Ginger Alden, Elvis's girlfriend at the time, who found Elvis dead in the upstairs bathroom. So, she certainly was upstairs after (and, in fact, during) his death, though his death had not yet been 'officially' declared.
After finding Elvis apparently dead, Ginger immediately summoned Joe Esposito (Elvis's friend and bodyguard), Al Strada (also Elvis's friend) and Dr. George Nichopoulos (Elvis's personal physician). Whether or not any or all of those summoned went upstairs is unclear. However, one can safely assume that at least Dr. Nichopoulos went upstairs, because of his reported efforts, however futile, to revive Elvis. (It was also Dr. Nichopoulos who declared Elvis dead.)
In addition, the Memphis Fire Department rescue unit arrived about half an hour after Ginger found Elvis. (Evidently, she summoned the others mentioned above *before* the rescue unit was called.) Because it was the rescue unit that rushed Elvis to the hospital, the paramedics had to have gone upstairs. So far then, we have quite a few people who likely went upstairs.
Further, right after Elvis's death, his daughter, Lisa Marie (then aged 9), reportedly immediately telephoned Elvis's ex-girlfriend, Linda Thompson. Yet, whether or not Ms. Thompson went upstairs (or even came to Graceland) is unknown.
Since Elvis's death, the upstairs of Graceland has been strictly off-limits. Many celebrities have requested private tours but all have been declined. Even President Clinton was refused. However, actor Nicholas Cage, a lifelong Elvis fanatic, "managed the impossible" by marrying Lisa Marie Presley, Elvis' daughter. His secret visit to the upstairs took place during the week of the 25th Anniversary of Elvis' death. [http://www.linkydinky.com/graceland/ ]
So, if your music teacher is asking "who were the people allowed upstairs ON THE DAY of Elvis's death," there likely were *more* than three people.
If, on the other hand, your teacher is referring to the *years* following Elvis's death, then the most likely answer is Nicholas Cage.
[Sorry to be so long-winded, but I wanted to be sure I covered all the bases. Hope you get the extra credit!]