I think that an email or a letter to the principal is a good idea. I would re-state very clearly what you want (math subject acceleration) and your supporting data. Then I would state that you do not recall consenting to your son's participation in a study but that it may have been in the morass of paper that you got at the beginning of the year. (This may provide them a way to save face). Either way, you are withdrawing your consent at this time. Given that your son will no longer be proceeding with the study, you would like to address implementation of math acceleration at your next meeting.

You may want to consider whether to cc people in the district who are higher up in the food chain. Like it or not, bumping it to a higher level may be necessary in your case.