I get the impression from a number of comments in this thread that quite a few folks seem to think that people drop out of school primarily because they aren't being "challenged" enough, or in the right way.
In my experience with folks who have dropped out of school (and unfortunately, given my upbringing, I've got a lot of experience with folks who have dropped out of school) I've found, or at least it's my opinion, that the overwhelming majority drop out because they don't really care for being challenged, period.
There are a great many people who are simply averse to applying themselves to hard work in any of its many manifestations and they simply don't have the discipline to put in an effort now in order to derive benefits down the road.
And it isn't just a problem in terms of education and the workforce.
Just look around you. Nearly 70% of Americans over 20 years of age are overweight with just under 36% of those being obese.
Americans don't have the work ethic and discipline required to eat right and exercise any more than they have the work ethic and discipline necessary to apply themselves in school, go on to institutions of post secondary education and repeat the process, and then go on to graduate school.
Many Americans who do apply themselves in school do so along the lines of the easiest and least rigorous courses of study.
Americans, by and large, are just lazy.
I know there are exceptions to this rule, as it applies to high school drop outs the same as it applies to anyone else.
I have one friend who is a high school drop out who went on to get his GED then went to trade school to learn how to be a Harley Davidson mechanic and now owns his own business. I have another friend who dropped out and spent a great many years apprenticing as a tattoo artist and he, too, now owns his own studio. I have another friend who dropped out of school and is now a First Sergeant of an Infantry Company.
But for every one of those successes that I personally know, I also personally know several others, and I'm sure there must be dozens or hundreds of others that I don't know personally, who find it an insurmountable struggle to simply show up at work early every day, stay late every day, and bust their ass every waking minute they're there - never mind doing anything "exceptional" to differentiate themselves from the herd otherwise.
Americans by and large want to punch a clock, be on location from 9 to 5 (and not one second longer), do as little as they possibly can while they're there (just enough so as not to lose their job), make zero effort to continue their education on their own time and dime in a manner that would be beneficial to their employer, and rush home in order to lay their fat ass down on the couch, eat a burriot, drink beer, and watch other men play sports on their televisions.
The "high school drop out" subset is just the laziest of the lazy.