Good money in it! I used to go to a dollar store once every other month and then sold it in school, the kids I didn't like I would charge more, once they bitched I went "I have the only supply, and you are demanding it...".
It's great money, I did it in 7th and 8th grade. Made a ton of money eight grade. At most 1 cent per candy and sold it for 10 cents on average. 25 cents for a piece of gum and people would buy that? That's some stupid school you go to.
I like Orbit personally, could get a good deal on it at BJ's Wholesale.
And yes it's against the school rules to sell anything, and it's against every school I know rules to have gum. Plus a store has to have a set price for the gun. I would make 500%+ profit on candy, the stores aren't making close to that, because there is a big supply outside of jail.