I'm not sure how to advise you about the hearing, other than to point out that you were, in fact, sober, and offered to prove it... Is it actually a crime to be under 21 and in the presence of alcohol even if you're not drinking?
Don't stop wearing your letters because of this incident. I'd even suggest a pin attire day - all greeks dress up and wear their pins on the same day.
Hang in there.