These studies are bullshyt.
I went to an education seminar at the University of Kansas decades ago. And in case you aren't familiar Kansas is white and conservative.
These are narratives of studies, they are studies of narratives.
But they often don't designate what qualifies as homework and what doesn't. Just because a black kid reads for 1 hour after school doesn't mean he's given the same credit as a white kid who does 100 algebra problems. If a black kid and an Asian kid both have the same assignment but the Asian kid finishes completes the assignment 11 minutes and the black kid finishes it in 18 minutes. The black kids doesn't get credit for having "spent more time doing homework".
Studying and homework are not interchangable terms yet they are largely interchanged. Studying might be reviewing an assignment/questions/reading, researching for ones own edification, whereas homework is an assignment that needs to be turned in and graded.
**Homework is a something to take with a grain of salt because not all homework is assigned equally. There's nothing in schools curriculum that states a student must have so many assignments to take home/complete at home. And not all classes are equal. A student may finish his algebra in class and never bring the book home, whereas the calc student might bring the book home everyday. Also most studies have some form of study hall/study period. Not only are two high schools in the same district/school system may have the same curriculum, but two different school systems have different classes, different curriculum.
This matters because while there are state standards that everyone has to meet, the difference is how they get there. For instance my high school had 8 periods like 40 minutes each. Whereas the school I wanted to go to had 4 classes roughly an hour and 50 minutes late.
2hrs doing math in class vs 45 mins doing math, or English or history is a vast difference. One school is going to have more "homework" but the other school has more "class time" each school can argue which makes for a better student.
12th grade English probably doesn't have as much homework as English AP. Biology 3. Might have more homework than chemistry 1.
What's the duration of these studies? Is this over one week? One quarter? One semester? One year? 2 years?
Who's reporting the studying and who is verifying it? Is it the students? The parents? The teachers?