As a young Oxford don, I was chairman of the panel assessing the preliminary examination that students took at the end of their first year. We failed the son of a particularly noxious third-world dictator. There was an opportunity for him to resit at the end of the summer. I waited in trepidation for a threat or an offer from a distant land. None came. Presumably the tyrant dealt with his son's problem in a more appropriate way, since the boy passed on the second occasion.
Like most teachers, I received the occasional token gift from a grateful – usually foreign – student but never anything of significant monetary value. No one has ever suggested I might receive a bonus on the successful completion of a student's course; although the effect of high examination grades at a prestigious university on lifetime earnings is extremely large. And, like most teachers, I would have felt insulted by the proposal if it had been made.
Many teachers would say that such a bonus would not affect their behaviour. I doubt this is true, and it certainly would not have been true of me. If the bonus were large, I would have kept a particular eye on that student's progress. I would have taught to the tests, ignoring any broader educational objectives.