The federal motor vehicle standard for overall vehicle width is 8'6" or 102" but my understanding is that school buses specifically are restricted to a smaller 8' even or 96". Therefore I imagine you have up to three inches per side to work with without question. Note this does not include things like mirrors which extend beyond the vehicle body but pertains to the body construction itself. I imagine as creative as skoolies are we'll find some gray area to fudge, like hinged porches or planter boxes or what-have-you.
While I like the concept of a roof deck or storage rack, two things specifically discourage me from considering one. First, it's a lot of weight way up in the air. Regardless of how lightweight your construction, ultimately you're adding mass above the roof which is going to have a significant impact on the stability and aerodynamics of the vehicle when in motion. And if it's storage then all that cargo is further compromising the performance characteristics from the original design. This is one of those factors that makes insurance companies frown on school bus conversions because I think in their collective minds skoolies all look like RAGBRAI buses. Second, the school bus is one of the safest constructed vehicles on the road because of the one-piece roof bow rib construction but that applies to accident survivability which doesn't necessarily translate into awesome long-term load-bearing capacity. A roof deck I can forgive because it's only carrying load while the vehicle is parked but rooftop storage to me is just an accident looking for a place to happen. It's basically the same conundrum we face when considering lifting the roof because we intend to compromise the original design with no way to adequately prove the reinforcements we add even match nevertheless exceed the strength of the original design. Raising the roof like roof racks also alters the handling and aerodynamics so things like the suspension and steering components all begin to come into the equation... But too often as DIYers we don't know what we don't know so we just do it and hope for the best.
Sorry to ramble, just getting it out of my system.