They might use home-school or other programs (small class sizes, private schools, tutors, etc) as controls.

If their assertion is that the current form of primary school is causing creativity to decline, they only need to demonstrate that it causes more decline than alternatives.

I'd have to dig up a reference to the study, but I won't be able to do that for the next few days. Google scholar returns various results for "school and creativity decline" if you have access to the relevant journals.

Self-replying because I cannot edit: see [1] for a link.

[1] https://news.ycombinator.com/item?id=11360290

So you have no clue? I suggest to you that part of not being a mindless drone is recognizing that correlation is not causation. And part of helping others not be mindless drones is not quoting small parts of studies and then making wild claims that the study may or may not have shown, counting on the fact that the vast majority of "mindless drones" are going to take your statement at face value.