If I’m Underage and Alcohol Is Served Do I Get Arrested?

Yes.

It is illegal for any person to sell, serve, or permit the serving of alcoholic beverages to a minor. The presence of alcoholic beverages and minors (under 21) in the same room gives rise to probable cause to make an arrest.

All the prosecutor has to prove is possession, either constructive, or actual.

The only exception is if you are under 21 and working in an establishment that serves alcohol: you can serve it.