Safety considerations mandate bus replacements at certain intervals. Once a bus is "Miled out" they are typically traded in for fractions of a penny on the dollar. 
Maybe ask yourself what you would do if you were on the school board and assigned to the transportation committee. Are you going to be the guy that says the bus is perfectly fine and we should continue running it until it fails catastrophically? Most school boards in rural areas, suffer from lack of fools willing to serve on the board. Maybe it's your turn?