In my experience the issue is with medium density cities and suburbs, which account for the bulk of the US population.
In the rural area where I grew up, and the few others I am familiar with, the school bus picked all students of all ages at once, because the routes were up to 25 miles one way and it was more efficient to drive them once rather than three times.
In the rural area where I grew up, and the few others I am familiar with, the school bus picked all students of all ages at once, because the routes were up to 25 miles one way and it was more efficient to drive them once rather than three times.