According to Jalopnik, these metal “rub rails” that run along the sides and back of the bus function as an extra layer of protection for the thin walls of the vehicle. They help absorb the force of a collision and also prevent a car from impacting the whole side of the bus in a crash.
Rub rails are also positioned at specific spots. A school bus driver known on YouTube as ClawBoss explains in the video below that the lowest one is installed at floor level, the middle one corresponds to the bottom of the seats, and the top one usually matches the height of the top of the seats and/or the bottom of the window. This strategic placement could prove useful after a bad crash if the school bus doors and windows can’t be reached or opened. Rescue teams would have a better idea of where to cut into the sides of the bus, and they’d also be able to tell where the impact occurred. If it’s below the bottom rail—i.e. below the floor—bus passengers may have avoided the worst of the crash.
Placement, size, material, color, and additional details about rub rails vary by state. In Illinois and Maryland, for example, they have to be “glossy black,” but other states allow them to match the rest of the bus. The bus’s color, by the way, is called “National School Bus Glossy Yellow,” and it definitely was chosen to be eye-catching. In 1939, a group of engineers decided that particular hue was easiest to see during morning and evening bus rounds.