Almost younger students improve themselves to get these awards ;-)
Just try and see what may happen.
I worked out 3: for good behaviour, for high participation and for 100% test correctly fullfilled.
If your class is very big, and you are lucky to hand out a lot of rewards, just let your students cut their reward out of the paper. So you save some time.
It worked quite good in my class, although I had to modify the established guidelines more than once. But even the youngsters (13 aged) liked to receive them, which was surprising me.