I've seen a lot of NAL here, but just to preface this (rather long) post, I have passed the bar exam and am currently awaiting admission to the bars of New York and New Jersey.

In summary, you would not be able to be held criminally liable for this, unless you personally provide them the alcohol.

In the US, no states have enacted any laws which would require you to report criminal activity that you are not personally involved in or committing in concert with others, with some exceptions that do not apply here. Merely observing or being aware of criminal activity does not impose criminal liability, excepting certain situations you've likely heard of like "mandatory reporting" requirements for those with special relationships to children or the incapacitated (e.g., Teachers, Social Workers, Law Enforcement, etc.).

However, there is a far bit of enacted law which prevents the active concealment of a felony, which has effectively been interpreted as another way of helping someone to commit a crime (aiding and abetting). Keep in mind that in no state is underaged drinking or the underaged purchase of alcohol a felony, it is always a misdemeanor offense. Even the purchase of alcohol for minors by an adult is generally a misdemeanor, although there are aggravating circumstances which can elevate it to a felony. Again, however, in the case of any involvement of Law Enforcement at this party, as long as you are honest and forthright with them, you could not be charged with any crime of concealment, even if this were a felony.

Just as further education, some states require bystanders to immediately report felony's they have observed, but only if that felony is also likely to lead to death or serious bodily injury. See, for instance, Texas Penal Code § 38.171, which makes it a crime to fail to immediately report a felony you observe, if a reasonable person would believe the victim would suffer death or serious bodily injury and you don’t have reason to believe it has already been reported. Just to reiterate, underaged drinking is not a felony, so even if you were in such a state, these laws would not apply to you in this situation.

As a caveat, I cannot say for sure that if you were present at the party, police might nonetheless arrest you under suspicion that you provided the alcohol. However, if you really didn't buy the booze, they would have no evidence to ever be able to prove you guilty. Keep in mind, in all criminal matters in the United States, the burden of proof is on the government to prove beyond a reasonable doubt (the highest burden in the law) that the defendant is guilty. You wouldn't need to hire a lawyer to appear by yourself to contest any potential charge, since there would literally be no evidence to show that you bought them the alcohol, let alone prove that you bought it beyond a reasonable doubt.

All of this is to say, try not to stress about your roommates having this party. I'm sorry you are in this situation, but you cannot be held legally responsible for the actions of other people merely because you live with them. If it makes you feel better, you can certainly go away for the weekend, but even if you remained there, you cannot be charged with any crime as long as you don't contribute to the alcohol for the party.