I've seen this!
I rode the bus throughout most of my school years including high school, up to my Sophomore year. (From then on, I caught a ride with my friend Brandon. I'd take the bus only in the event he wouldn't be able to drive.)
Anyway, every bus driver I had would develop and use their own individually styled wave. However, there was one particular driver I had, name of Ted, who was a bit.....unbalanced. Whether a career of driving a school bus created this unbalance, I don't know; though it seems as good an explanation as any. Ted, a man in his mid to late fifties, was an odd man who, in his infinite wisdom, thought it appropriate to seperate the males from the females on the bus. (Females occupy seats one through ten, males in the back half.)
Ted also didn't like me very much, and he went out of his way to try and make my bus ride as miserable as possible. Usually, if I was causing some sort of disturbance (or if I wasn't!), up to seat #1 I went, right behind the keen eyes of his 4 foot mirror. A position such as this may seem precarious to most, and that may be true as evidenced by one of Ted's more ridiculous diciplinary writeups on me. He decided to "write me up" for tapping my foot; I had to use the bathroom pretty badly, but he didn't accept that as a valid explanation. The assistant principal promptly tossed the writeup in the trash upon my arrival in his office, but I digress. This vantage point did give me a unique view into the habits of this particular bus driver.
Passing other school busses on the road is an everyday, inevitable occurance. And yes, bus drivers do have their little Secret Bus Driver Waves. But Ted's wave was different. In fact, it wasn't really a wave at all.
Ted would see an oncoming bus, his cue to point his index finger straight up until he was within eye contact of the other driver. As soon as that happens, he'd flip his finger forward to point at the other driver, as if to say "YOU THE MAN!"
Ted would do this like clockwork. We all found it humorous, but that didn't discourage him one bit. As I said, Ted was a strange man. A reliable driver, somewhat likable, but very strange.