here's some data on the most popular candies around the world: http://images.businessweek.com/ss/09/06/0624_worlds_best_selling_candy/1.htm
M&Ms are #1 in the US.
and here's specific info on top 25 in the US:
Gum is surprisingly popular so I would recommend selling some gum varieties in addition to (edible) candies.</body></html>
I would take the most popular, and take a note of your inventory. Count everything you have, and make a note of that. Sell those for about a month, and look back on your data and see what sold the best. When you have that info, then you can then purchase more of those specific candy bars than the ones not selling so well.
Also make sure you school allows you to sell these, you don't want to get in trouble!