School buses aren't built with ride quality as a priority. In fact, school buses are very noisy things to ride in on the highway. that bus load of kids screaming at each other....they're not just trying to talk over each other, they're also talking over the bus noises. So, even if you are not concerned with floor insulation as a thermal barrier, consider it as a noise barrier. You'll be glad you did.
On my bus, I left the original floor in place at the right (front side) of the steps over to the driver's seat base. That little bit of floor can't be walked on so when I finish that area, I'll apply a sound deadener over the rubber matting and then install my carpet.
The floor at the top of the steps, I have sound deadener directly on the metal, 1/2" plywood which will have carpet on it eventually. The driver seat actually sits on a raised platform, when I replace the seat, the platform will get sound deadener and a new heavy rubber mat to replace what was there originally.
With the floor insulation I have installed in the "camper" area so far has made a very noticeable difference in the noise coming from the engine.