The short answer is yes they could potentially still charge you. I don’t practice in your jurisdiction so I couldn’t tell you how likely it would be one way or the other (I’m inclined to say based on the offense that you wouldn’t be charged). What I could advise is that you do not say another word to any law enforcement officer without the assistance of an attorney. Oftentimes the police may not have sufficient evidence to proceed and the client talked his/her way into being criminally charged. In the larger scheme of things, the mere purchasing of alcohol for a minor isn’t the case of a century. If you are charged with this offense, an attorney may very well be able to obtain a favorable disposition for you that won’t affect school/career/future. Good Luck.
You can be charged with supplying alcohol to a minor up to 18 months after the date of the offense. However, it is unlikley that anything will be done if it goes past 30-60 days without any charges being filed.
Every case varies depending on the law and the facts. Because questions often fail to disclose important facts, these 'answers' cannot and should not be relied upon as a replacement for a full or complete review of an actual case by a retained attorney. It is axiomatic that one only 'gets what they pay for', and this is one of those situations where such a phrase generally applies.
Its possible that they will charge you later, no one can tell you for sure. Unfortunately, all you can do is wait. If they call you bring a lawyer.
The first thing you should do is never EVER again lay out all your facts in an online message board. This forum is not confidential and you never know who may be watching. You should discuss your entire situation in private with a criminal defense attorney. While you may be safe, it would be far better for you to benefit from the skills and experience of an a attorney in your area.