I can recount for you the story that I've heard from those in my area who have kids:
First, the most dedicated full-time parents decided to pick their kids up and drop them off, so that they would have more time with their kids and the kids would spend less time in transit.
Then, some unruly kids caused some trouble on the buses (the district isn't going to station a warden on the bus, and the driver has a route to keep), so some more parents started driving their kids. Now it was super-involved parents and also parents of bullied kids.
A huge fraction of kids get bullied, so now we're looking at a bus filled with the lower half of the parental protectiveness bell curve. Obviously that's doing to distill the troublemakers, further lowering the level of protectiveness a parent would have to have before taking their kids off the buses.
This forms a vicious cycle. The good kids are no longer diluting (and providing social pressure to) the bad ones, and the buses gain a (deserved? I have no way of knowing) reputation for being where bad parents dump their kids to be tortured by other bad kids.
Now there's a social stigma. The set of parents that drive their kids now include the very-involved, the protective, and the responsive to social stigmas. So, finally, the children of the poor (whose parents don't have the means to fall in to one of the above groups) are stuck alone with the bad kids - and now we're at our present state, where cars line up around the street and everyone is spending an hour trying to pick their kids up from school.