Two things :

They are teenagers. They don't think, they're a hormone soup bouncing from one fad to the other. We were all reckless I'm sure during those soup times.

And

Most of the products already have a tamper proof packaging, since the scare of decades ago over people fucking with over the counter medications and foods.

With that in mind, the best solution would be to make these kids walk around the store with a billboard around themselves saying on either side "please check the products and produces for signs of tampering before purchasing". Done, two birds, one stone.