After seeing the 12th grader again vaping in the bathroom, you mention how you did research on those and you found some facts that they should know. You tell them all about the negative effects of e-cigarettes and why teens are using them more and more. You explain the harmful ingredients and why how advertising markets teens.
Once you explained all this, the 12th grader looked mortified. They didn't know that e-cigarettes were harmful and could effect their body negatively and they threw the vape in the trash, thanked you, and walked out.
Every time you saw that 12th grader in the hall, they nodded their head at you and gave you a smile. You helped them stop a bad habit and they are grateful. Now hopefully they will pass on that information to anyone who needs it too.