Honor Code (at least at USAFA): We will not lie, steal, or cheat, nor tolerate among us anyone who does.
Drinking underage is not an honor code violation since you aren't lying, stealing, or cheating. Just like speeding is not a violation or going AWOL or fraternizing.
Now if someone asks you if you have alcohol in your room and you lie to them, then that is an Honor Code violation. Or if someone ask you how old you are before they give you alcohol and you lie about your age to get the alcohol that will also be a violation.
Too often on this forum, posters believe that every time a cadet does something against regs, that they are breaking the honor code. This is not true! Every year, several cadets graduate that are triple centurions (someone who marched over 300 tours for breaking the regs) but they haven't broken the honor code. They just break the regs a lot!