Well the 6 student rule sounds like a good idea, make sure to have a camera and every aisle and the row of all the drinks, kids will not want to steal as easily if they are being watched. Even check the tapes later for the kids, just so you can be ready next time they come in.
Of course, keep the candy bars and stuff that are easy to steal in your visible range, so keep candy in the aisle in front of the register at the counter and try to have energy drinks view, maybe close to the candy because I know when my friends steal from places (I don't do it so often anymore) they always go for the energy drinks.
That was a long sentence..