The standard color for school buses, often called "school bus yellow" was set as a standard when determining bus construction standards.  The color was chosen because black lettering on that shade of yellow was easiest to see at dawn or dusk.  There have been safety advances through the years with the automatic stop sign and safety arm that keeps children from crossing in front of the bus in the driver's blind spot.
 
The argument for further upgrades and safety systems comes down to money and whether the benefit of spending school funds in that area is more beneficial than spending it in other areas.