'get in/on" and "get off/down'

It's another one of those questions where either set of answers is correct, I'd say (remember, there's more than one way to say almost anything in probably every language on earth). You get down from a bus because the floor is usually pretty high and you have to come down steps to hit ground. But for some reason I don't think you'd "get up" onto a bus. Get on or off work too.

I might rephrase it as, "When the bus arrived, the door opened and passengers got in at the front, while departing passengers got down from the rear of the bus".

Actually I'd probably use "got on.off" in preference, but your example is okay.

. . and all this is not to be confused with "getting it on" and "getting off" - both sexual terms.

Ride in/Get out of- small vehicles like cars, taxi, etc.
Ride on/Get off- big vehicles like ship, airplane
*exception= for vehicles that you can't go inside like motorcycles, bikes, etc, use get off