Do you think that putting a gifted or maybe HG+ kid in average classes and pretending she isn't gifted is reasonable in any circumstance if she performs better there?
Ouch. No, I don't - but it's the "pretending she isn't gifted" bit that makes me react that way, and my guess is that you wouldn't actually do any such thing (even if the school did). If she were happy in average classes *and* were getting appropriate intellectual stimulation outside school, I would think that was OK. It does seem ridiculous, though.
