One of the reasons I do not like to give out rewards to students is not only the cost that comes out of my own salary, but also the fact it actually discourages students to study over the long run. A lot of teachers seem to believe in it, but there is data now mirroring my observation.
If a student is already interested in the material, giving rewards is likely to make them less interested, because rewards are an obvious kind of way teachers will try to control student behaviour. If students are not that interested, they will likely cheat in order to get the reward. Of course, if rewards are given out all the time, the rewards then become an expectation of students, not a bonus at all.
Too bad my bosses at school don’t get this.