You know that it's illegal to drink alcohol if you're under 21 in Virginia--or any state, for that matter--so you can get a minor in possession charge, which is commonly called an MIP, if the police catch you, give you a breath test, and determine that you're drunk. When police bust a high school party, for example, they'll often just line people up and give them all breath tests to determine what citations are needed.
However, it's important to note that you don't necessarily need to be drunk to get an MIP. The legal limit for driving is 0.08 when looking at blood alcohol concentration. If you blow a 0.01, though, you may not even be feeling the impact of the alcohol, but you could still be charged.
The idea is that you're not supposed to have alcohol at all if you're under 21, so even the slightest amount on your breath shows that you broke the law. You may feel quite sober, but police can still point to this reading to give you the citation. This is similar to the way that commercial drivers can sometimes be given DUI charges even if they're not over 0.08, as they are held to a higher standard than other drivers.
Additionally, these are possession charges, so there are situations in which you could be give the citation even if you're not drinking yet. For example, if the police catch you buying alcohol or transporting it to a party.
Remember that the overall goal of an MIP charge is often education and community service, so the ramifications may not be as dire as you think. Be sure you know all of your options.
Source: FIndLaw, "MIP: A Minor in Possession," accessed May 13, 2016