During the Jail Break mission you have to drive a prison transport to the prison. When you reach the prison, a gate will open. Move in between the gate so that it can’t close, making sure not to completely enter. To your left there’ll be a grey school bus marked “Prison Transport System”. Get inside it and store it at a garage. Now return to the prison and into the transport that’s still in the gate. The mission will then proceed normally. Note: This is the only opportunity to get the Prison Transport Bus.