Chiming in a little late here -- this thread should be stickied with the title, "Advocacy 101"!
I'm thinking that whatever assessment they gave him probably supports her position that he's not that smart, and possibly not even gifted. Obviously we know that's bollocks as DYS doesn't fly willy-nilly with their admissions policy.
However, *she* doesn't know that, and whatever assessment they gave him might have caught him on a bad day. When my ds-then-5 took his first-ever computer assessment (and yes, it was the MAP), he did fine but not great -- 76th percentile, I think. We were told that, regardless of his private 99.9th IQ and WIAT scores, he needed to be 4 RIT points higher in order to join the gifted first grade math class. So, sorry, no differentiation.
When he took the MAP again a month and a half later with the rest of his class, his score jumped to the 99th percentile, numerically better than the average autumn third grader ... and they conveniently "forgot" to mention that to us. We didn't find out until the school year was almost over. Amazing. That was the final straw that told us that the school just wasn't interested in working with him, and why we pulled him for private gifted.
I honestly wouldn't bother with standard deviation discussion, although I do like the "If I had a child with an IQ of 50 instead of 150 we wouldn't have waited to have this discussion." The standard deviations/private testing don't seem to mean much to many schools. If they have his private test results in hand -- not to mention spending every day with him for the past 6 months! -- and they're still trying to tell you he's "not that bright" ... well, they're not going to suddenly get it.
Have you talked to your DYS person for ideas or help? Isn't that what they do -- advocacy assistance?
Above level testing -- if he took the MAP as an assessment, you can simply norm his score against the curve for older students. I also agree that giving him an end-of-year assessment for, say, second or third grade might be an eye-opener for them. I think that's a great idea and wish I'd done it!
Good luck. I really hope you're able to convert her.