Having been in that situation myself in my late teen years, I would suggest that you report it to the police. Perhaps you may be able to report it to the police anonymously if you do not want the other people from the party to know it was you who reported it. Let me give you an example of why:
In my first semester of college, I suffered severe consequences because I did not report underage drinking. I would try to socialize with other people who lived on my floor, and sometimes I would be talking to a group of people in someone’s room and people would start drinking. I would excuse myself after a few minutes and return to my room, but other people who lived on that floor always saw me leaving and thought that I had been drinking with them. One night, because of a medical condition I’ve had since elementary school, I was vomiting in the bathroom, and someone reported me to a resident assistant. The RA didn’t call the police that night, but if he had I would have been clear because they would have found that there was no alcohol in my body. However, the RA did report it to his supervisor the next day, along with the fact that he saw me coming out of rooms where he suspected drinking several times. Although I told the truth to the supervisor when he called me down for a meeting, the supervisor said that he could not believe me because there was too much evidence against me.
My reputation was completely damaged, even though I didn’t actually participate in the drinking, because people started calling me an alcoholic, I lost all my friends on that floor, I was given a warning notice that I would be kicked out of the dorms and that it would affect my academic records if there was any more evidence of me drinking, and the school notified my parents because of a loophole in the law, which means my family looks at me as a liar and they think i have a drinking problem now. If I had called the campus police, people would have avoided me and called me a snitch, but I would have had immunity from any legal or academic consequences. I can tolerate people not liking me if the reason is because I stood up for what was right.
