Why there are no seatbelts on a schoolbus

In an interesting OpEd piece in the New York Times on rationing health care, he is making a point about the cost worth spending to save a life, and refers to

"Twenty years ago, the National Research Council, an arm of theNational Academy of Sciences, examined a proposal for installing seat belts in all school buses. It estimated that doing so would save, on average, one life per year, at a cost of $40 million. After that, support for the proposal faded away. "

http://www.nytimes.com/2009/07/19/magazine/19healthcare-t.html?pagewanted=3&_r=1&em