A gifted kid is at least 2 standard deviations above normal, whereas a kid 2 standard deviations below normal is considered developmentally challenged.

Ask them if they would try to teach a kid 2 SDs below normal with the same curriculum they use for normal kids? Then, ask why it would make sense to use that same curriculum for kids 2 SD above normal?

Maybe a graph to give them a picture.

What I am is good enough, if I would only be it openly. ~Carl Rogers