It jumped out at me that you wrote that you "heard" that every third grader failed the math test last year. If you don't know it for certain from the school system itself, find out -- don't go by what other parents tell you. They may indeed be right, or....not. If there was a high failure rate on one portion of a standardized test, it can indicate many things and does not necessarily mean the math instruction is lousy. Our state is revamping its (dreaded and useless but required) standardized state tests and we've already been warned that the changes mean there will be a lot lower scores and more failing ones this year as the tests "shake out the bugs" etc.
Also, don't let standardized testing scare you too much, no matter what. It is used to grade the schools and the teachers but not the kids (unless it's something like SATS much later on!).
You said you are looking at a charter school --are you not interested in the local public regular school? Is there a specific reason? Check out all the options before assuming charter is better -- if that's what you're doing. The "gifted and talented" track in the regular schools could be as good or better than a charter for her (that is the case around here). She would be too young for most GT programs in first grade but many start by third.