The deployable stop-sign device mounted on the side of many school buses is called a stop arm.
A stop arm is a pivoting, usually octagonal sign that folds flat against the bus when not in use. When the driver opens the door to load or unload students, the arm extends from the left side and displays a red stop sign, often with flashing lights or illuminated lettering.
Its purpose is to warn traffic approaching from behind that the bus has stopped and that vehicles must remain stopped. In North America, the sign is designed to control traffic in both directions, although exact enforcement rules vary by jurisdiction. Some buses have a second arm near the rear.
A stop arm is not the same as a crossing arm. A crossing arm is a separate barrier that extends from the front bumper to encourage children to cross several feet ahead of the bus, where the driver can see them. “Warning bar” and “rub rail” are not the standard names for the stop-sign device.