As disneymarie can probably attest, Pennsylvania has what is called a "constructive possession" law meaning that a minor can be cited for possession of alcohol if you are present in a situation where alcohol is readily available and you made no good faith attempt to distance yourself from it. It applies if there is no barrier limiting access to alcohol in a confined or limited area.
NOTE ALSO: A person arrested for driving under the influence has a right to a blood alcohol or breath test. THERE IS NO SIMILAR RIGHT under citations for underage drinking, since the law deals with purchase, attempt to purchase, consumption, transportation and possession of alcohol. Breath and blood alcohol tests are irrelevant to all of these except, perhaps, consumption.
This information is from my kids' college alcohol policy. So bottom line is in PA, yes you can get cited for possession if you are underage and at an event where alcohol is readily available (like most college house parties).