I don't have nearly enough evidence to have high confidence in this conclusion, but I think high schoolers are intellectually lazier simply because of the selection effect of the college application process. I have noticed (warning: anecdotal evidence!) that college students (myself included) tend to procrastinate a lot more, but that may only be because they have increased freedom to do so.

But I haven't seen any reason to believe that laziness is more prevalent in high school than in the general population.

And you might be right--I don't think it's an easy task to convince the general population to think, either. (Plus, "general population" includes people who don't have a high school education.)