Cans of Coke during summer was always a great one. The school canteen could never keep enough drinks in stock so we took up the slack. Used to work out at about £10 per case profit. You could easily shift 10 cases a week. The company that ran the canteen hated us, but even the teachers ended up buying from us!
I also managed to buy teachers editions of several text books off eBay... Now that was profitable.