I say start with the principal or assistant principal, whichever is better for you, but make sure the transportation department is involved. I think the bus driver's supervisor is in that department and the principals have limited influence. When I had a bus driver problem, I talked to the elementary school principal who called the bus company on the phone while we were together. The principal meant well, but he didn't get hold of quite the right person nor was a fully prepared to take advantage of a three-way conversation.
And I do think that just making the driver/bus company know that you are watching might temporarily improve the situation. When/if it regresses, you do it again. Short of her losing her job or getting re-assigned, this improvement/ regression may be a pattern, but if you are the squeaky wheel, hopefully most of the time the driver will stay on the improvement side and that may be all you can achieve.
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9