I haven't read the other responses yet, but I will.
My 8 yo (third grade) believes 100%, and so do most, maybe all, of his friends. The school he goes to has a culture of Christmas, and Santa. They host a breakfast with Santa every year, and the kids love it. Many of the teachers have writing assignments involving holiday traditions, and Santa is often mentioned.
However, I encourage my son to respect other's beliefs in all things. This has come up many times with religion. We're Catholic, but most of our friends are of other faiths. I encourage both of my sons to learn about other religions, and they enjoy talking about our faith as well, but I don't want them to force our faith on others. I've had the same discussion about Santa. Different people believe different things. Right now my youngest son believes in Santa, but he knows that not everyone believes. He's not going to argue with anyone about it. He's just going to happily believe (for now).
When/if he asks me about Santa, I'll be gently honest. My 15yo figured it out on his own after Christmas in the third grade, but didn't say anything to me until the fourth grade. He took it well, and has enjoyed continuing the tradition with his younger brother.
Good luck with your daughter. :)