According to wikipedia at this url: http://en.wikipedia.org/wiki/Truck_classification
, class 8 is any vehicle with a GVWR of greater than 33,000 pounds. It specifically doesn't indicate a number of axles.
I wasn't able to find a primary source to verify this information with a quick search of the US DOT FHWA website. However, the information hasn't changed on that page in quite a while, so I tend towards trusting it.
I believe in the OTR trucking world, there is the axle distinction due to the fact that they're all 5th wheel rigs. There are limits to the amount of weight allowed on a single axle, and other factors at play. I'm far from an expert on that subject though.
Now, with all that said, let's get to the idea of pulling a 5th wheel commercial trailer with a school bus.
There's a conversion on this site (5th wheel hauler I think?) where a conventional bus had the body shortened and a 5th wheel hitch installed to the frame so that it could haul a 5th wheel trailer directly. I see no reason why a bus so modified couldn't *technically* tow a commercial trailer from point A to point B as long as the transmission was rated for the actual weight involved.
A competent and licensed air brake technician should be able to install a set of helping hands on such a setup. Of course, the bus would have to be air brake equipped, and all the standard trucking safety issues would have to be addressed. Also, since you would be doing this for hire, you would need a CDL with air brake endorsement. Otherwise, you would just be in a world of hurt should anything go wrong (including a check by law enforcement).
I would personally recommend against this. I think it would be a lot of work to convert the school bus for this. I think you would still end up with DOT regulation issues, even with a CDL and air brake endorsement. Plus, it would require you to have a CDL in the first place.
If I had a bus modified to haul a 5th wheel RV, and I needed to move an empty commercial trailer a few miles, I probably would just do that trick where you lock the air brakes open, and tow it brakeless very slowly to the destination. I think of a few miles as less than 20. I would also avoid doing this anywhere near a police station, frequent patrol area, or weigh station, or congested traffic areas (ie, anything other than the country).
One possible loophole I just thought of is the farm use trick. If you could go that route, I believe you could avoid the CDL. You'd have to slap farm use tags on the trailers as you towed them, and be careful of your trip radius. I'm pretty sure this would work here in VA, but I don't know if your state includes similar exemptions for farm use vehicles. Also, you'd have to keep the fact that you were moving the trailers for hire a secret, which is difficult to do. Businesses have to keep track of all expenses, and paying you for moving the trailers would leave a paper trail, which could come back to bite you.
Again, I don't advise doing this. Hopefully this information will be helpful and perhaps spark more thought and discussion.