As it has been said already, there really isn't any sort of standard ground clearance for buses. For one thing different states have different spe'c's for how deep the skirts have to be. And some schools or operators just pay the extra $$$ for the deep skirts which for some reason the number 27" sticks in my head.
Ground clearance is also different when you are talking about approach and departure angles or how much clearance do you need to keep from getting high centered.
The low point of most buses will usually be either the engine/transmission oil pans or the bottom of the rear differential. You can't attach stuff that would hang down that low on most buses as you would get high centered in the middle.
Also, with a bus with a front engine you usually have a lot of bus sticking out behind the rear axle. You can't hang stuff very low back there or you will be scraping it off trying to get in or out of curb cuts.
I think, as a general rule of thumb 20" between the axles is a doable number. Behind the rear axle I would think 14" would be about the maximum doable number.