The only indication we have of the school's fault is the unsupported allegation of the mother - which was essentially denied by the school. We don't have enough facts to determine what really happened.
Our schools make sure that the young bus riders get on their buses, so where was the school when this child should have been placed on the proper bus? If they weren't making him walk, they should have been making sure he got on the proper bus.
We also have the _fact_ that the 5 year old walked 1.5 miles (or so) in 20ish temps (per another poster) without any assurance that the kid knew the route.
Schools are responsible for putting the kid on the right bus. The driver left without the kid so the facts are that somehow the school sent the kid out on what would have been about a 30 minute walk, unsupervised, unexpected,and quite possibly unrehearsed.
The relevant school personnel should be eternally grateful that it wasn't my kid.