I am a mandatory reporter and I have, and will continue in the future to report distribution of alcohol to those under 21. Even on purim.
Why are you granting such a status to secular law? Mandatory minimum drinking ages do not work and are fundamentally flawed ideas. Driking alcohol for religious reasons especially is not a crime. It is an observance of our religion. Do Christians moser on children who take wine at communion? No. Why should we be turn in yiddishe teens and bochurim who want to be b’simcha on Purim? Is that such a heinous crime in your book that you want to turn people in and expose them to the cruel horrors of the criminal justice system, potentially giving them criminal records that will ruin their chances at getting jobs, loans, degrees, etc., all because they wanted to drink? How is this justice? How is this halachic? Instead of coercion, try using education- explain how drunkeness and alcoholism are sinful in the eyes of halacha, unhealthy, talk about Korsakeff syndrome, etc. You get farther in life when you make heads meet, as opposed to making heads knock.