I agree that its very plausible that grades affect self perception. However, I have low confidence that it would be more predictive than IQ.
Same could be said for conscientiousness, but thus far it seems like the behavioral testing and the reports from grade-naive "homeroom" teachers seem to increase the correlation rather than decrease it compared to self-report alone.
I don't have strong opinions on the subject, but I wouldn't necessarily expect uniform results across the IQ spectrum. It might well be that different things are more predictive at different ends of the IQ curve.
In particular with respect to IQ and conscientiousness, it seems to me that at high IQ levels the "necessary-for-A+" IQ tops out and conscientiousness starts to dominate, while it's the reverse with low IQ -- if you're just not smart enough, conscientiousness won't help much.
The soon-to-be-resigning Dominic Cummings, advisor to the Education Secretary of the Coalition government, has released a 250-page manifesto describing the problems of the British educational establishment ("the blob" in Whitehall parlance) and offering solutions. I post this here because both his analysis and recommendations are likely to be interesting to LW, in particular an increased emphasis on STEM, broader knowledge of the limits of human reasoning and how they relate to managing complex systems, an appreciation for "agenty"-ness in organizational leadership, whole-brain emulation, intelligence enhancement, recursive self-improving AGI, analysis of human interactions on a firm evolutionary-psychological basis, and a rejection of fashionable pseudoscientific theories of psychology and society. Relevant extracts: