Minor in Possession of Alcohol in California

My question involves criminal law for the state of: California

A couple nights ago some friends and I met up with some people. They were over 21 and had asked if they could drink in our parked car while we waited for the rest of our group. We agreed. They got out to find a restroom and shortly after a cop came up and shined their flashlights in the car and asked us all to get out. We cooperated. The 3 of us in the car are all over 18 but under 21. We were given tickets for minor in possession of alcohol, however, the cops gave us back the alcohol which included a 1.75 liter of vodka and an open cup with vodka in it. I'm just wondering why they would give us the tickets, only to give us the alcohol back when no one over 21 was present? Will this fact possibly help me in court? Also, what happens if i plead guilty? If i want to plead not guilty do I need a lawyer?

Never allow people to drink or otherwise possess open containers in a car, parked or otherwise. You were EXTREMEMLY LUCKY you weren't charged with more things.
Always plead not guilty unless given counsel to the implications of doing otherwise. If you want to beat the charge (of which you appear to be patently guilty of) you will need a lawyer.

The fact they returned the alcohol was stupid, but doesn't mean that you are suddenly not guilty of the offense.

The specific penalties will depend on the specific statute you were cited for. There could be at least 4 state offenses that might apply, and any number of municipal codes that might apply.

What is the code section on the cite? And if not a B&P (business and professions Code) or VC (Vehicle Code) section, what city were you in when you were cited?