Welcome to skoolie.net. If you click on User CP in the upper left, you can fill out your details and then everybody will know where you're at and what kind of bus you have - which is relevant to the issue of how to demo your school bus sign (and most other issues as well).
If you have the same type of school bus sign as me, you need to dig an opening somewhere on the gasket with the utility knife, then grab one side of the cut with a pair of pliers and twist. This should be enough to get the gasket started out, then you can pull it out all the way around easily. This is also a handy trick if you ever want to break into a bus via the rear windows, which are usually gasketed the same way.
I don't know where you'd find an exact replacement gasket for this or how you'd reinstall it, but if you're doing a ticker tape kind of thing you might just want to start from scratch anyway.
I'm dealing with the weirdness of trying to frame and insulate the cab by building an insulated bulkhead wall behind the driver's seat, so the cab area will not be part of my living space and I can leave it pretty much as it is (other than fixing the big holes in the floor there right now). This will give me better insulation and I will also be able to lock up my living area separately, for if I ever have to leave the bus for a while at a mechanic shop.