The chains at the bottom of school buses are there for traction such as in snow, ice etc.
The chains are basically automatic snow chains that are connected to a motor and drop down and wrap around the tires when the bus driver pushes a button in the bus.
When the bus becomes stuck in the snow the bus driver just presses a button and the chain system lowers then starts spinning the chains underneath the tire.
The chains then help the tire gain traction in the snow.
The bus driver doesn't have to get out and put snow chains on the tires because they can easily push a button to operate the snow chains.
Fire Trucks as well ambulances usually have those automatic snow chains as well that operate the same way and even semi trucks.