It's NOT a "shame on you" as another poster said. Yes, kids can learn at any school (or not in school at all), but if she's topping the expectations with all A's, she is doing the best she can with what is presented to her, and you are doing the best you can with helping her study what is being presented to her. Ask your neighbors? The ones that are going to the same school already? That answer was off base.
Your child made A's at a school where the standards were lower and easier. It's like that all the way to university levels---you can go to a public university with ease, or really have to make a mark to be accepted into some private universities. That's what you're paying for in a private school: the edge, to have more doors open to you. An example even at my preschooler's level: there are preschools where you can be real proud to send your child to kindergarten knowing how to write their name, the alphabet, and some colors. Other preschools are a lot more rigorous and will teach them how to read and some math by the time they go to kindergarten. Still others will have them beginning conversational Spanish. My son is a smart, engaging, funny, happy little guy....well behaved, just started pre-K and is already beginning how to read. But I shopped one place where the cow giving me the tour sneered and said "You do realize this is for ACCELERATED students, right?" I said "That's the third time you've said this, and you have not even spoken to him yet. Please enlighten me on how you can judge him on anything but height, until you let him play with something, take a test, or at least converse with him." She couldn't answer, and when I went on my tour, there were NO classes AT ALL going on (I had specifically requested to see class when making the appointment). She said the entire school was out because of swine flu, so I said "Interesting. The non-"accelerated" schools I've shopped know how to maintain hygiene and don't have an epidemic". We didn't go there since her attitude was gross......I can't imagine my son having "fun" there, which is what he should be doing in the preschool years. (Er, I know this went a little off track, but that was an example of a private preschool that obviously had some pretty high standards, off base or not, they thought my son might be "behind" and he's far from that).
Don't sweat this. Just realize that you've put her in a new school with higher expectations, but when you get her caught up she will have some great college testing and a good school for her college "resume".