I'm not suggesting "stupifying" them, but rather educating them. Forcing them to ask an adult to get the Plan B for them so there is a bigger chance of them getting information about it rather than allowing them easy access. If it is really allowed OTC, then there is no one required to tell them any side effects of it, at all.
Information is the best protection. This is not the way to spread information about the best methods of protecting from unintended pregnancies.