Hire a hundred extra staff to check in every kids luch as they come to school and then stand behind them during lunch to make sure they don't trade?
If kids these days are anything like I was when I was in school they will go out of their way to trade lunches just because the "system" says the can't?
My kids can't trade their food, because I say they can't AND it is a school policy. My children attend a Jewish private school. We have a kosher kitchen and we keep kosher. Though most families send kosher food with their children for lunch, not all is prepared in a kosher kitchen. My children cannot swap food because of the differing religious standards, and I would not be a happy girl if the school were to allow trading of food. I give my children what I want them to eat.