Surely behavior of children on a busses plays _some_ role in this decision. Not sure how they would even gather stats on this but busses in my youth were total zoos. Further, schools near me won't allow many children to walk home even within a safe walking distance with parental consent for (and I quote) "liability reasons". Absurd.
Where I am the school line is mainly to prevent kids from running between the cars; anyone who has been authorized can just be let free and walk home - some of whom just walk a block to where their parent is waiting.