Here's my two or three cents on the subject:
Doesn't middle school bring enough trauma? I mean the poor kid is going to have to deal with her body changing, new expectations and feelings, why throw in losing the magical part of childhood that is Santa?
I knew, deep down, that Santa wasn't real around your daughter's age but I didn't want to grow up... and so I acted as though I still believed. I am so thankful my dad was zealous to keep that alive for me. My most vivid memory is one Christmas Eve asking him and he assured me Santa was real. Well, this might be have been a good time to have some of the other conversations others have advised (and I think they are really good options when I child comes out and asks). I didn't really believe him until the news came on that night and ... on the radar they reported a sleigh with what appeared to be reindeer in the sky. Well, that really helped me believe for another year because ... it must be real if it's on the news! Well, I am so thankful to have had another year.
Teach her to be confident ... if it comes up she will know that some kids may not believe and that's their choice. But she shouldn't feel bad because she does.
As parents we couldn't tell our kids when to walk or talk or anything else... we were forced to wait until they were ready... regardless of what the "charts" say is normal. I say let her decide when she's ready to give up that precious part of childhood. There's so much of growing up that we have no choice about, especially at that age.
Best of luck to you. Hope you have a Merry Christmas this year.