ergot: Been there, done that!
As a retired teacher I can truthfully say that the biggest deterrent to a child's academic success is dysfunctional parents who fail to properly motivate and work with their child. Of course, as a teacher I did not use politically incorrect terms such as "dumbshit" in reference to these parents and I was always very polite to parents who deserved that title, but it was incredibly frustrating to deal with these people since they, as tronagirl pointed out, persistently failed to see what should have been incredibly obvious. As for the debate regarding nature versus nurture and concepts such as learned helplessness, I regrettably agree with what ergot said. Although there are occasional hopeful exceptions, as a general rule, the apple does not fall far from the tree.
Early in my career I leaned more towards the nurture side in the nature versus nurture debate, but eventually I came to realize that genetics was the greatest single factor in determining the success of a child. Our one size fits all system (aka, mainstreaming) is not the answer. Children need to be supported at their own levels and mixing up a bunch of kids with ridiculously discrepant intellectually ability makes it almost impossible for a teacher to effectively met the needs of his or her students. Smaller class sizes with ability grouping and exclusion of disruptive and dysfunctional students is the answer. Unfortunately, politically correct California is moving in the opposite direction and the results have been abysmal and will only be getting worse.