"Pursuant to these requirements, federal safety regulations applicable to bus manufacturers have been developed. There are a total of 37 federal motor vehicle safety standards that apply to school buses. Many of these also apply to other types of motor vehicles, but several (among them standards 131, 220, 221, and 222) are written specifically for school buses."[1]
As for seatbelts,
"Large school buses are heavier and distribute crash forces differently than passenger cars and light trucks do. Because of these differences, bus passengers experience much less crash force than those in passenger cars, light trucks and vans."[2]
It seems that even weight[3] is a safety consideration.
[1] https://www.cga.ct.gov/2004/rpt/2004-R-0730.htm [2] https://www.nhtsa.gov/road-safety/school-buses [3] http://www.americanschoolbuscouncil.org/love-the-bus/educato...
Buses are designed for many years of service with long-lived and easy to replace parts. On the safety front they are painted yellow and have some lights. That's pretty much it. They don't even have seat belts.
It looks to me like efficiency trumps safety in this case.