Unfortunately I would say you'd be safest contacting the FMCSA to get a clarification (http://fmcsa.dot.gov/).
I'll take a stab if you'd like, but I'll need to know more information...
I think the bus rules are for "passenger carrying" buses, but I'm not sure how or if that would apply to you. Are you "for hire" or do you haul the same people around all the time?
Also, do you go more than 100 miles out from your home terminal?