12 ounces of what?
If you drank 12 ounces of beer then no, it would not show up
If you drank 12 ounces of vodka, then yes it would most certainly show up (although you probably wouldn't be able to show up yourself)
Figure that it takes about 1 hour to metabolize 3/4 of an ounce of alcohol from your system. 12 ounces of vodka could take 9 hours. Remember, the school is not looking to see that you are below .08. They are looking to see if you show up with any reading above .00