Yes, but that's at the radio -- in the front, not back where the people are who want the volume adjusted. And you can't just put in a pot or cutoff switch back there, because the radio power amp transistor can burn up if there is no load on the output.
The passengers can just ask the driver to change the volume. I don't think it will be an issue as the volume on the boom box we used in the past didn't get changed all that much.
When you say speakers in the back, I think of the rear half of the coach. Maybe you mean the front half, just behind the driver.
I don't think I would want to be shouting from one end to the other, over road noise, "A little louder . . .no, too much, a little lower now . . ." That would be too much like backing into a small space while someone you can't see is shouting "This way . . .now go that way a little . . !"