You're trippin' into a very grey area with the CDL, temp tags, and such. The real problem is that the rules are not uniform throughout the U.S. and it's hard to tell what an Officer of the Law would do in any particlar area or circumstance.
In my (this time humble) opinion if the the bus still has the commercial (school bus) plates on it from the State of purchase and the seats are all still in there it's going to be a chore to explain that you are *not* driving a commercial vehicle requiring a CDL. That Officer *may* choose to understand the situation with your bill of sale and such but then again...
If the plates came off the vehicle and you had a trip permit or better still temp tags (or best of all non-commercial plates) you could probably make the trip without much trouble; especially if most of the seats are removed (get under that magical 15-passenger count).
So, if Mass has no temp tags what happens there when you buy a new car? Do they have plates on hand so you can drive home immediately?
Do you know anybody in another State that can get the bus registered as a non-commecial vehicle (private truck, suburban, private bus, etc)? Then you should be able to get the bus registered in Mass without too much hassle since you bought it from a private party with non-commercial plates. Then after you get it converted you could change it to a motorhome.
Initially the key here in Washington was just to tell them I had a bus that I wanted to register as a private vehicle; I didn't go the motorhome route. At least here, there's nothing to prevent you from licensing a bus just like a private truck as long as you don't exceed the passenger and weight limitations for commercial vehicles. Once the bus is legally an "RV" you sidestep those issues. [Wouldn't you know it, right after I registered the bus they changed it so I could register as a motorhome without the inspection! Which I did! (thanks J.B.)]
Cover your nether regions on insurance for sure; you can get insurance on a private bus from GMAC insurance (GMAC is also available through Camping World and Good Sam (at perhaps a better price)). Make sure the bus is *not* still licensed as a commercial vehicle then GMAC just needs the pertinent info (VIN, Year, Make, etc) to get the policy in force.
So here's the YMMV part! This all worked for me and I never ran into any problems at all; most importantly I was never driving a "commercial" vehicle and a CDL is *NEVER* required on a non-commercial vehicle (that's Federal Law). That doesn't mean various States can't require extra endorsements to your regular non-commercial license though so be careful. If Mass is a State that requires an endoresment for Air Brakes (even on non-commercial licenses) and you buy a bus with air brakes make sure you get that endorsement before you start driving home. Some States require an endorsement (again, on a non-commercial license) for "extra large" vehicles so do your homework. And if the bus exceeds private weight limits in your State and can not be licensed as a private bus/truck/suburban you're outta luck without getting it licensed as a motorhome (that's when the Federal exemption kicks in on the CDL requirement).
Hope some of this rambling helps....