I'm not sure why you're concerned that you hadn't challenged her before kindergarten. I mean... she's five, right?
The school has to see the evidence that she's capable of doing more challenging work, and they're going to ignore it if it comes from you, because they assume all parents are prejudiced. So if you want them to start taking her seriously, you need to have her tested.
As for talking with her, girls typically dumb themselves down in order to fit in socially, so what you're seeing is totally normal. I'd start with talking with her about how totally cool it is to be smart.
