How do bus doors lock?

In a school bus and I'm assuming it's the same for city buses, grey hounds etc, the "front door" automatically locks once the bus is started and it has enough air pressure to keep it closed. You can bleed air pressure off to allow it to be opened from the outside say of your going to leave it running and step outside real quick. After that it can also be locked from outside using a key. The back door just has a bar controlled by a simple lever and cannot be unlocked from outside.