Non-specific information like "strong reader and good at math" doesn't communicate anything useful. How strong? How good?

You're right that they won't trust your assessment of her grade level, but you can provide specific examples that can clue them in to the fact that she's not at a beginning-of-first grade level. Some examples might be:

Math
- Tells time to the nearest minute on an analog clock
- Adds and subtracts numbers with more than two digits, correctly utilizing borrowing and carrying
- Can count to 1000 by 1s, 2s, 5s, and 10s.

For reading level, you can list some books she has read aloud.

I didn't find anything like this on the Colorado DOE site you linked, but Louisiana has a fairly comprehensive listing you can use as a guide to help you write these out, as well as come up with your own fairly well-informed assessment of just what her grade level might be: http://www.doe.state.la.us/topics/gle.html

Ultimately, the school has to do some of their own assessments, and they're going to need time to complete them. This is because you're a parent, and as woefully lacking self-awareness is in humans, awareness of the abilities of their own children is even worse. The school has dealt with enough parents with misplaced ideas about the superiority of their offspring that they're not going to believe you. They will believe their own testing, and you can augment that belief with your own, concrete information.

How much time is enough time? I'd let your DD determine that. If she seems comfortable in her school, then some patience is called for. If she starts displaying behaviors that are a cause for concern, then it's time to kick the school in the rear.