I would give the principal one more shot, and then move up the chain of command. I say this because it seems pretty clear the principal just made this up on the fly (or is clueless).
A bigger concern to me is the conflict of interest between getting more money for the school and the needs of kids.
I would, in writing, communicate something like this.
"I am writing once again to address the issue of our request to have DC [accelerated to 3rd grade or whatever]. Based on our conversation [or email or letter] of September X, 2010 you indicated that such a request was impossible because of a grant provided to the school by the Gates Foundation for the [blah blah] study. According to you, the Study does not allow [blah blah].
I am deeply disturbed by the position you [or the school] have taken. If indeed the Gates Foundation study does not allow children to move classrooms, then the district has created a serious conflict of interest in which additional funding and/or payments to teachers are given priority over the right of a child to an appropriate educational environment. In addition, my child's inclusion in the MET study is in violation of informed consent laws, as I am quite certain that i have never given consent for my child to participate in a study which would so limit his/her educational option.
If my understanding of our prior conversation is incorrect, or if I have misunderstood the school's relationship to the Gate's Foundation study, I look forward to your clarification. "
I would send this letter before your meeting to Principal and I would hope for a reply before that. Also, do not go to the meeting with the Principal alone. Take a spouse, partner, parent, friend or someone, and take very good notes.
I would definitely, however you word things, make sure you give the principal a way to back down on this. Be willing to politely say with a smile "I am sure I must have misunderstood your position as I am certain the school would never sacrifice a child's right to an appropriate education in order to pay their teachers $1500 more per year.
I mean really, if there was a major bullying situation, they wouldn't be able to move a kid from a class?