Typically the bus has a stop sign with flashing lights that automatically swings out when the bus driver opens the doors. The law in most places in North America is that it is illegal to pass the school bus when students are embarking or disembarking, so in practice it is safer for the students to cross the street while the school bus is there.

Image of a typical North American school bus: https://en.m.wikipedia.org/wiki/School_bus_traffic_stop_laws...