It's up to the judge and what the law states wherever you are as to what will happen to you.
As for them just "banging down the door", you were upstairs and don't know for certain what happened downstairs when they came to the door. If they saw a violation of the law through the open door, they then had a legal right (based on the U.S. Supreme Court use of evidence in "plain view") to enter the house. And if they kept seeing more and more violations (i.e. - underage drinkers), they would keep searching.
Their evidence: underage CONSUMPTION just means that you consumed alcohol, not that you were drunk or intoxicated. Whether you were falling down drunk, just had a beer can in your hand, or just had a smell of alcohol on your breath, that is usually enough to charge you with it.
For those who say they officers probably won't show up in court, that's a load of crap. Sure, once in a great while, something will make an officer miss court. But the vast majority of the time, they will be there, for two main reasons. If they have to do it on their time off, they get paid overtime to be there (extra money for them), and most departments will take internal disciplinary action if an officer just doesn't show up, unless the officer has a really good reason.
My advice is to get a lawyer, or attempt to talk to the public defender ahead of time. Try to arrange for some community service or a small fine if you go ahead and plead guilty. But if you want to fight it, go ahead. There is always at least a chance you could win, but the punishment is usually a little tougher than if you plead guilty. Either way, for a couple of minor violations, I doubt the judge will give you a really harsh punishment.
And about the one person who said juvenile records are sealed and don't worry about it: 18 is an adult. You lost your last chance at a free pass on your last birthday.
- Police Officer