What's your relationship with the principal like? If it is really good, it might be worth requesting another meeting and talking through the presented options. (I'm basing this on your comment that he's read the research and acted accordingly.)
We found that having outside data helped us. YMMV. In your case, it sounds as though it might be really helpful to have test results showing where your daughter is at in math. (Objective tests, not the one that she might be given again at the school.) Also, be aware that some educators think a passing score to move on needs to be 100% in these circumstances. We were fortunate that the educator who gave end-of-year tests to our DS thought that 80% was the bar. (DS scored higher than that, but the tester said if it was in the 80s, we would discuss a plan to accelerate while we would fill in "gaps" at home.)
You may also find that test results could qualify your child for gifted programs (weekend camps, etc.) outside the school district.
