My anecdotal evidence runs opposite yours, and I suspect the people you are thinking of are either both smart *and conscientious, or happen to be in one of those rare academic setting which favor intelligence over conscientiousness. Or maybe your perception of peoples intelligence is influenced by how academically successful you know them to be. Who knows how perceptions of intelligence map on to IQ and GPA conscientiousness when humans use funny metrics like facial symmetry, clothing, facial expressions, eye contact and other forms of nonverbal body language?
My opinion isn't just based on anecdote, though:
Various measures of self discipline twice as predictive of GPA as IQ in adolescents - and you'll notice the importance of IQ really drops off after the middle quintile.
Self-Reported Conscientiousness predicts college GPA better than both SAT & High School GPA
Summarizing from my general impression of things I've read, IQ is generally more predictive than self-reported conscientiousness for grades ...but the difference is way closer then you'd expect for a self report going up against a cognitive test.
a lot of us can attest
Hmm...so you're drawing from the LW demographic? That's gonna be a higher IQ group, but they'll also have other unusual traits...I'm kind of curious now. Good candidate question for the next census...
Edit: Here's a poll with lots of relevant data (on GPA , IQ, and procrastination of Lesswrongers). Analysis not geared towards our specific question. Raw data is available. Eyeballing it, LW GPA's tend to be 3.3-3.9, IQ's range 130-160. Apparently IQ correlates with absolutely none of these things...which is actually not that unexpected, since everyone is in the higher range and they probably didn't all use the same test. But still, it doesn't seem like LWers are skating through school...just hovering above average. And this is just the out of the people that volunteered the self-report GPA. Self reported procrastination in childhood did correlate with self reported low high school GPA,
http://lesswrong.com/lw/7s4/poll_results_lw_probably_doesnt_cause_akrasia/
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: