Yes a really "dumb" kid will still be "dumb" academically, but people here are severely underestimating what money can buy in education. I've seen really "dumb" kids be brought up to the level of "passably smart" simply because of resource access.
When you have teachers who are not overworked and are paid lovely salaries (ie 6 figure salaries), imagine every teacher you have actually cares and invested in your education? Imagine every class has recordings of every lecture as well as a curriculum that has a lot of forethought and time put into it? And imagine if the school has a lacking teacher, than the parents hire some ex-olympian, or ex-superstar coach or teacher to make up for that? I've seen parents hire ex-olympic athletes to get some untalented rich kid to close to olympic level.
Honestly, I think it's really understated how much educational spending can affect a kid. Most people will not see what I've seen, and even from someone who came from a middle class background who used to believe wealth does not matter when it came to education, even I was shocked by what money can buy in "academic success" and my opinion changed. I at this point, don't need to see a study because it was that drastic.