In order for the car to have the same mass as the school bus, the car would need to be made of a material that is denser than the material the school bus is made of. For example, if the car is made of lead and the school bus is made of aluminum, the car could be smaller than the school bus and still have the same mass.
In order for the car to have the same velocity as the school bus, the car would need to be travelling at the same speed and in the same direction as the school bus. For example, if the school bus is travelling at 50 km/h and the car is travelling at 100 km/h, the car will have twice the momentum of the school bus.
If the car has the same mass and velocity as the school bus, it will have the same momentum as the school bus. This means that the car will have the same inertia as the school bus, and it will be equally difficult to stop or accelerate.