My question involves criminal law for the state of: California

I was in Santa Barbara for a sports game last weekend. I ended up at a private party (only friends were allowed inside the house) and received an MIP within the property.
After doing some research, here are some details that I think will help me in the case:

-I was not drinking, nor was I cited for drinking or being intoxicated.
-I was in the process of filling a red plastic cup with beer from a keg... for another person.
-Like I said, the party was private. MIP description: “any person under the age of 21 years who has any alcoholic beverage in his or her possession on any street or highway or in any public place or in any place open to the public is guilty of a misdemeanor.” I was not in any place open to the public.
-An officer grabbed me while I was filling the cup (as it, it was halfway full).
-I was respectful and compliant.

Other things that may or may not help:
-I drive a service vehicle for my job.
-This is my first offense of any type.
-It occurred outside of my hometown.
-I'm a good student, volunteer frequently, etc.
-I turn 21 very soon (days after the court date).


What do you recommend that I do?
Should I plead not guilty because of the "private place" discrepancy,
or should I plead guilty with these facts to try to lessen my punishment?

Is there any way I can avoid losing my license? I know there are alcohol classes and things that are offered sometimes.


I appreciate any help.
Thank you.