When must a school bus be marked with orange school bus signs?

You answered correctly. In Germany, a school bus must display the orange school bus signs whenever it is actually transporting school children. This helps other road users recognize the vehicle’s special purpose and drive more carefully near it. The signs do not need to be shown before or after the children are on board; they are only required during the transport of school children. Remembering this helps you identify when extra caution is expected around school buses.