That one bothered me right off the bat because the bus driver behind would have seen it drive out of the bank and cut in line AND he has a radio connecting him to the bus barn and the other buses.
Actually, it only just now occurred to me that the Joker could have bought-off, intimidated, killed, or otherwise replaced those bus drivers ahead of time. I also suspect that he didn't stay in that bus very long.
Yeah, I never assumed he stayed on the bus for very long. All he had to do was get a couple blocks away from the bank and he could have hopped into another vehicle.
Although, as with other scenes, I find it ironic that The Joker talks a lot about "not having a plan" and making it up as he goes, and yet pretty much everything he does in the movie requires a TON of pre-planning for it to work (like the school buses being there at just the right time).
Unless he's just got really good instincts or something.