18 Year Old Possession of Alcohol

My question involves criminal law for the state of Virginia.
My friends and I were caught smoking weed in a parking garage. The police searched us and our two parked cars, claiming their probable cause was the smell. While the parking garage was private property, one of the reserved spaces belonged to one of the drivers with us.
While I wasn't charged for the marijuana, I was charged with underage possession of alcohol.
My first question is, was the search they conducted legal? Does smell constitute probable cause in Virginia and/or were we technically guilty of criminal trespass?
Second, if it was legal for them to search the first car, was it also legal for them to search the second? Considering that the only paraphernalia in use was in the first car, and the smell was obviously only being generated by that?
Also, what consequences am I likely to face as a first-time offender? Is there any way to keep this from going on my record? I understand that I may not be convicted if I agree to attend an "educational program."

Absolutely the smell of illegal substances is grounds to initiate the investigation of criminal activity. Use of the other senses is permissible as well. The use of hearing to respond to gunshots or someone screaming for help for example. The use of eyesight to observe shoplifting or signs of domestic violence is another.

Okay, thank you for the answer to my first question.

I am assuming you and your friends were hanging around BOTH cars, so yes, they had the right to search it.

The fact that one car was in a reserved spot has no influence. Marijuana is illegal, no matter where you smoke it.

Can anybody tell me what consequences I'm likely to face?

You might well consider a lawyer. Many of the local courts (especially ones around universities) will have a deferral program that if you do community service, take an alcohol class, etc... and avoid a criminal record.