***** NOT A TEACHER *****
Hello, Albert:
If you use "on a bus," it will probably always be considered correct.
A few books say that some people prefer "in" only in this very unusual kind of sentence (which I have made up):
"We were traveling on a bus. It started to suddenly snow very heavily. The driver announced: 'Sorry, folks. I have to park the bus on the side of the road until the snow stops falling. You will all have to sleep in the bus tonight.' " (My note: in other words, the bus becomes something like a hotel room. And we always say "in a room.")
James