I think the limit to be on the road is 13'6". What about rv parks and national parks, and other places we might like to stay for the night? I know the national parks have a limit of 27'. What about a height limit for the vehicle?
If your question is about raising the roof, I think you might be best off by raising as much as you need, but not perhaps as much as is legally possible. Consider that you might want to put something else on top, a deck or solar for instance, but also consider how out of proportion a School bus looks with a very high roof. If you intend to take it on back roads keeping it as low as you can will help with branch and low bridge clearance.
Driving my 10'6" tall bus in normal neighborhoods and around town is a chore due to the height. Low branches everywhere and Comcast lines as well.
Raising the roof to drastic levels is going to hurt drivability a good bit.