While aerodynamic teardrops are most efficient that means nothing if the up front cost is prohibitive.
Also, efficiency and a box that's strong enough to appease the "think of the children" types are very much at odds with each other.
For a bus with low mile requirements some boxes to hold batteries and a big DC motor where the engine goes would work fine.
The "scaled up golf cart" architecture works well if the route does not change and is very cheap to implement.
On your second point, a bus doesn't necessarily have to have a heavy steel frame for it to be safe. There are plenty of lighter weight (e.g. composite/carbon fiber) solutions which don't sacrifice safety.
Sure, if the mileage requirements are low then there's definitely a cost-effective retrofit solution. But if you're a company already building a full electric bus and investing a lot of R&D into the pack design anyway, then you could probably just sell battery solutions to school bus companies as another revenue stream. I'd wager, though, that the larger addressable market is customers looking for new electric buses for longer routes.
[1]http://www.columbia.edu/~ja3041/Electric%20Bus%20Analysis%20... [2]https://www.nrel.gov/docs/fy16osti/65274.pdf
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.
"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...
I say all of this more to make the argument that rather than retrofit old buses, in my opinion it makes more sense to outright replace them. Right now Proterra is the largest domestic producer of pure electric buses. They compete on most contracts with BYD in China, and New Flyer in Canada. Behind BYD and the government, the electric bus market in China has absolutely exploded over the last several years. Proterra is anticipating high growth in the U.S. market as well and just launched a new factory in LA last month [2].
[1]https://www.proterra.com [2]https://www.forbes.com/sites/alanohnsman/2017/07/26/proterra...