(All that said, I do believe there is a genetic component in whatever it is we mean when we say “intelligence”–although we definitely mean many different, unrelated things to different people–in addition to the significant impact of parenting, home life, and of course education. The last item is of course the most easily controlled, so that’s the one I and most others focus on, and much of the research suggests that the single most impactful variable in education outcomes is teachers.
A great teacher can boost a kid 2 grade levels in a subject and a poor teacher can drop a kid a grade level or two, so that’s a tremendous factor.
I’d also argue that students who work hard often end up being pretty “intelligent” by whatever metric you use, because some of the few things we know about academic “intelligence” is that it’s related to acquisition and application of knowledge, that it’s not a permanent, set quality, and that it can be nurtured–or, of course, squandered.)