Hi Tiz,
I will pass along the advice that our school gave to us and our son during the grade acceleration process last year. They instructed our son to respond to the question of "Why were you allowed to skip a grade?" with the reply "That is what the school decided was best." Perhaps the same advice could be applied in your case? This makes the school the one that decides the issue. Let them deal with the other parents. Say that it is the school's job to decide what is best for each child, and that they must have arrived at this solution or plan after much careful review and research into the current educational advances. It is certainly easier than saying that your child needs it (which is true) but is subjective enough to allow the other parents to respond that their child needs it too.
You could always take the route of launching into a fascinating and very long-winded discourse of the current educational research until the poor soul's eyes glaze over.
That sometimes works as well, depending on how well you know the person (or want to know them in the future!)
