Well you can start off with small rewards, every week that a student can get a candy of their choice or a small treat, if they're doing really well twice or more up to the adult every month you could take them to the park or arcade, or if they want at the end of the month they could get a videogame of their choice but you should do the video game one after they finish the previous game you've gotten them.
Hope this is a good idea you shouldn't copy and paste this though best you can do is paraphrase this or just wait and compare different answers until you find the best one for you.