Originally Posted by Bostonian
According to your logic a score on an IQ test should not convey much information either, because it too is affected by the variables you mention. In fact, even a child who usually gets adequate

- sleep
- exercise
- healthy food

might take an IQ test on a day when she recently had not gotten enough sleep or eaten much (maybe reduced appetite due to illness).

Indeed. This is a feature of IQ tests that is well-supported by evidence.

Originally Posted by Bostonian
School grades represent work done over an extended period of time and average out some environmental noise, but of course not stable features of the environment.

School performance does not average out any "environmental noise" that is pervasive and omnipresent. If a child is regularly not getting the support they need from their environment, they will continue to underachieve.

For illumination purposes, I suggest you seek out an online forum where highly-involved and caring parents regularly discuss the lack of support in their schools and communities for gifted children, and all the negative consequences that can come from that, including underachievement. I'm sure you can find one somewhere.

And then consider what happens when parents are not highly-involved and caring.