During a fire drill in a school or other such public building, you and your significant other hide in a closet or somewhere small and deserted, then when the building is clear you proceed to have sexual intercourse, finishing as everyone comes back into the building and joining the rest of the crowd. If done correctly nobody will ever find out about your actions.
Matt: Dude where were you during that fire drill today?
Jeremy: My girlfriend and I did a fire drill quickie. We fucked in the bathroom while everyone was outside. It was epic.