Some school districts will train you for the CDL, and some even pay you while you're training (mine didn't at the time). My training didn't take very long because I already owned a used school bus and knew how to drive it, at least. You have to take four standardized tests (general CDL, airbrakes, passenger endorsement and school bus endorsement), have a driving exam, and take a 14-hour special school bus class (this is all in PA, at least). The standardized tests aren't super-difficult, but you do have to actually know the shit, you can't guess your way through it and hope to get a passing grade. I just crammed the free online tests over and over until I was getting 100% on them. I think it was maybe a month and a half between when I started my training and my first day on the job, but that was with not doing a whole lot each day. My district was (and still is) a weird combination of desperate need for drivers with a frustratingly slow pace of actually training people.

I have a class C. I wouldn't bother with a class B unless I actually needed it.