For what it's worth, there are many areas in the U.S. where the infrastructure would reasonably allow for this and it's more a question of whether the parents would allow it. Even where this isn't possible, it's very common for schools to offer buses from extremely near to their homes to the school, which is a process that as you have pointed out, parents do not necessarily need to be involved in.
Here in SF our kids have some of this independence; but their school is still miles away. I walked to my suburban school in the 70's. SF has density to permit some independence but then undoes the benefit of that with its schooling model and chaos.
Pre-pandemic my kids could take MUNI. I biked with them. The bike ride is not bad but is too dangerous for kids who have not yet developed paranoid city road awareness. So my spouse or I got with them every time..
But this is already an unusual circumstance. Among urban friends in other places I can't think of more than a handful whose kids were independent in this fashion.
Here in the Bay Area places like Fairfax let you live the dream though... it seems plausible that if the ground shift to "hybrid" or full remote is real and sustained, there will be even more reordering of lifestyle and population in pursuit of this.
